Re: [PATCH v3 3/3] fbcon: Add option to enable legacy hardware acceleration
On 2/2/22 12:05, Daniel Vetter wrote: > On Tue, Feb 1, 2022 at 11:52 PM Helge Deller wrote: >> >> Hello Daniel, >> >> On 2/1/22 21:11, Daniel Vetter wrote: >>> On Tue, Feb 1, 2022 at 7:59 PM Helge Deller wrote: Add a config option CONFIG_FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION to enable bitblt and fillrect hardware acceleration in the framebuffer console. If disabled, such acceleration will not be used, even if it is supported by the graphics hardware driver. If you plan to use DRM as your main graphics output system, you should disable this option since it will prevent compiling in code which isn't used later on when DRM takes over. For all other configurations, e.g. if none of your graphic cards support DRM (yet), DRM isn't available for your architecture, or you can't be sure that the graphic card in the target system will support DRM, you most likely want to enable this option. In the non-accelerated case (e.g. when DRM is used), the inlined fb_scrollmode() function is hardcoded to return SCROLL_REDRAW and as such the compiler is able to optimize much unneccesary code away. In this v3 patch version I additionally changed the GETVYRES() and GETVXRES() macros to take a pointer to the fbcon_display struct. This fixes the build when console rotation is enabled and helps the compiler again to optimize out code. Signed-off-by: Helge Deller Cc: sta...@vger.kernel.org # v5.10+ Signed-off-by: Helge Deller --- drivers/video/console/Kconfig | 11 +++ drivers/video/fbdev/core/fbcon.c| 39 ++--- drivers/video/fbdev/core/fbcon.h| 15 +- drivers/video/fbdev/core/fbcon_ccw.c| 10 +++ drivers/video/fbdev/core/fbcon_cw.c | 10 +++ drivers/video/fbdev/core/fbcon_rotate.h | 4 +-- drivers/video/fbdev/core/fbcon_ud.c | 20 ++--- 7 files changed, 75 insertions(+), 34 deletions(-) diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig index 840d9813b0bc..6029fd41d7d0 100644 --- a/drivers/video/console/Kconfig +++ b/drivers/video/console/Kconfig @@ -78,6 +78,17 @@ config FRAMEBUFFER_CONSOLE help Low-level framebuffer-based console driver. +config FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION + bool "Framebuffer Console hardware acceleration support" + depends on FRAMEBUFFER_CONSOLE + default n if DRM + default y + help + If you use a system on which DRM is fully supported you usually want to say N, + otherwise you probably want to enable this option. + If enabled the framebuffer console will utilize the hardware acceleration + of your graphics card by using hardware bitblt and fillrect features. >>> >>> This really doesn't have much to do with DRM but more about running >>> questionable code. That's why I went with the generalized stern >>> warning and default n, and explained when it's ok to do this (single >>> user and you care more about fbcon performance than potential issues >>> because you only run trusted stuff with access to your vt and fbdev >>> /dev node). >> >> I think this is something we both will keep to have different opinion about >> :-) >> >> This console acceleration code is not exported or visible to userspace, >> e.g. you can't access or trigger it via /dev/fb0. >> It's only called internally from inside fbcon, so it's independed if >> it's a single- or multi-user system. >> The only way to "access" it is via standard stdio, where fbcon then >> either scrolls the screen via redrawing characters at new positions >> or by using hardware bitblt to move screen contents. >> IMHO there is nothing which is critical here. >> Even when I analyzed the existing bug reports, there was none which >> affected that specific code. > > Maybe to clarify. The issues that generally result in this code going > boom in syzbot are when you race console activity (which can be > largely controlled through VT ioctls from userspace too, plus > /dev/kmsg) against fbdev resizing on the /dev/fb/0 nodes. The locking > there is kinda wild, and the code is very hard to understand. This is > why we've resorted to just disabling/deleting this code because most > cases we have no idea what's actually going on, aside from something > is clearly not going right. Yes, that's clear. But that (missing) locking needs to be on a higher level in the call chain, so the same lock fixes would fix scroll-redraw and hw acceleration. > Also the multi-user here means "you run untrusted code from other > people", not "you run multiple things in parallel" or "multiple people > are logged in at the same time". > >> Anyway, let's look at that part in your patch: >> >>
Re: [PATCH v3 3/3] fbcon: Add option to enable legacy hardware acceleration
On Tue, Feb 1, 2022 at 11:52 PM Helge Deller wrote: > > Hello Daniel, > > On 2/1/22 21:11, Daniel Vetter wrote: > > On Tue, Feb 1, 2022 at 7:59 PM Helge Deller wrote: > >> > >> Add a config option CONFIG_FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION to > >> enable bitblt and fillrect hardware acceleration in the framebuffer > >> console. If disabled, such acceleration will not be used, even if it is > >> supported by the graphics hardware driver. > >> > >> If you plan to use DRM as your main graphics output system, you should > >> disable this option since it will prevent compiling in code which isn't > >> used later on when DRM takes over. > >> > >> For all other configurations, e.g. if none of your graphic cards support > >> DRM (yet), DRM isn't available for your architecture, or you can't be > >> sure that the graphic card in the target system will support DRM, you > >> most likely want to enable this option. > >> > >> In the non-accelerated case (e.g. when DRM is used), the inlined > >> fb_scrollmode() function is hardcoded to return SCROLL_REDRAW and as such > >> the > >> compiler is able to optimize much unneccesary code away. > >> > >> In this v3 patch version I additionally changed the GETVYRES() and > >> GETVXRES() > >> macros to take a pointer to the fbcon_display struct. This fixes the build > >> when > >> console rotation is enabled and helps the compiler again to optimize out > >> code. > >> > >> Signed-off-by: Helge Deller > >> Cc: sta...@vger.kernel.org # v5.10+ > >> Signed-off-by: Helge Deller > >> --- > >> drivers/video/console/Kconfig | 11 +++ > >> drivers/video/fbdev/core/fbcon.c| 39 ++--- > >> drivers/video/fbdev/core/fbcon.h| 15 +- > >> drivers/video/fbdev/core/fbcon_ccw.c| 10 +++ > >> drivers/video/fbdev/core/fbcon_cw.c | 10 +++ > >> drivers/video/fbdev/core/fbcon_rotate.h | 4 +-- > >> drivers/video/fbdev/core/fbcon_ud.c | 20 ++--- > >> 7 files changed, 75 insertions(+), 34 deletions(-) > >> > >> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig > >> index 840d9813b0bc..6029fd41d7d0 100644 > >> --- a/drivers/video/console/Kconfig > >> +++ b/drivers/video/console/Kconfig > >> @@ -78,6 +78,17 @@ config FRAMEBUFFER_CONSOLE > >> help > >> Low-level framebuffer-based console driver. > >> > >> +config FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION > >> + bool "Framebuffer Console hardware acceleration support" > >> + depends on FRAMEBUFFER_CONSOLE > >> + default n if DRM > >> + default y > >> + help > >> + If you use a system on which DRM is fully supported you usually > >> want to say N, > >> + otherwise you probably want to enable this option. > >> + If enabled the framebuffer console will utilize the hardware > >> acceleration > >> + of your graphics card by using hardware bitblt and fillrect > >> features. > > > > This really doesn't have much to do with DRM but more about running > > questionable code. That's why I went with the generalized stern > > warning and default n, and explained when it's ok to do this (single > > user and you care more about fbcon performance than potential issues > > because you only run trusted stuff with access to your vt and fbdev > > /dev node). > > I think this is something we both will keep to have different opinion about > :-) > > This console acceleration code is not exported or visible to userspace, > e.g. you can't access or trigger it via /dev/fb0. > It's only called internally from inside fbcon, so it's independed if > it's a single- or multi-user system. > The only way to "access" it is via standard stdio, where fbcon then > either scrolls the screen via redrawing characters at new positions > or by using hardware bitblt to move screen contents. > IMHO there is nothing which is critical here. > Even when I analyzed the existing bug reports, there was none which > affected that specific code. Maybe to clarify. The issues that generally result in this code going boom in syzbot are when you race console activity (which can be largely controlled through VT ioctls from userspace too, plus /dev/kmsg) against fbdev resizing on the /dev/fb/0 nodes. The locking there is kinda wild, and the code is very hard to understand. This is why we've resorted to just disabling/deleting this code because most cases we have no idea what's actually going on, aside from something is clearly not going right. Also the multi-user here means "you run untrusted code from other people", not "you run multiple things in parallel" or "multiple people are logged in at the same time". > Anyway, let's look at that part in your patch: > > + bool "Enable legacy fbcon code for hw acceleration" > + depends on FRAMEBUFFER_CONSOLE > + default n > + help > +Only enable this options if you are in full control of machine since > +the fbcon acceleration
Re: [PATCH v3 3/3] fbcon: Add option to enable legacy hardware acceleration
Hello Daniel, On 2/1/22 21:11, Daniel Vetter wrote: > On Tue, Feb 1, 2022 at 7:59 PM Helge Deller wrote: >> >> Add a config option CONFIG_FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION to >> enable bitblt and fillrect hardware acceleration in the framebuffer >> console. If disabled, such acceleration will not be used, even if it is >> supported by the graphics hardware driver. >> >> If you plan to use DRM as your main graphics output system, you should >> disable this option since it will prevent compiling in code which isn't >> used later on when DRM takes over. >> >> For all other configurations, e.g. if none of your graphic cards support >> DRM (yet), DRM isn't available for your architecture, or you can't be >> sure that the graphic card in the target system will support DRM, you >> most likely want to enable this option. >> >> In the non-accelerated case (e.g. when DRM is used), the inlined >> fb_scrollmode() function is hardcoded to return SCROLL_REDRAW and as such the >> compiler is able to optimize much unneccesary code away. >> >> In this v3 patch version I additionally changed the GETVYRES() and GETVXRES() >> macros to take a pointer to the fbcon_display struct. This fixes the build >> when >> console rotation is enabled and helps the compiler again to optimize out >> code. >> >> Signed-off-by: Helge Deller >> Cc: sta...@vger.kernel.org # v5.10+ >> Signed-off-by: Helge Deller >> --- >> drivers/video/console/Kconfig | 11 +++ >> drivers/video/fbdev/core/fbcon.c| 39 ++--- >> drivers/video/fbdev/core/fbcon.h| 15 +- >> drivers/video/fbdev/core/fbcon_ccw.c| 10 +++ >> drivers/video/fbdev/core/fbcon_cw.c | 10 +++ >> drivers/video/fbdev/core/fbcon_rotate.h | 4 +-- >> drivers/video/fbdev/core/fbcon_ud.c | 20 ++--- >> 7 files changed, 75 insertions(+), 34 deletions(-) >> >> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig >> index 840d9813b0bc..6029fd41d7d0 100644 >> --- a/drivers/video/console/Kconfig >> +++ b/drivers/video/console/Kconfig >> @@ -78,6 +78,17 @@ config FRAMEBUFFER_CONSOLE >> help >> Low-level framebuffer-based console driver. >> >> +config FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION >> + bool "Framebuffer Console hardware acceleration support" >> + depends on FRAMEBUFFER_CONSOLE >> + default n if DRM >> + default y >> + help >> + If you use a system on which DRM is fully supported you usually >> want to say N, >> + otherwise you probably want to enable this option. >> + If enabled the framebuffer console will utilize the hardware >> acceleration >> + of your graphics card by using hardware bitblt and fillrect >> features. > > This really doesn't have much to do with DRM but more about running > questionable code. That's why I went with the generalized stern > warning and default n, and explained when it's ok to do this (single > user and you care more about fbcon performance than potential issues > because you only run trusted stuff with access to your vt and fbdev > /dev node). I think this is something we both will keep to have different opinion about :-) This console acceleration code is not exported or visible to userspace, e.g. you can't access or trigger it via /dev/fb0. It's only called internally from inside fbcon, so it's independed if it's a single- or multi-user system. The only way to "access" it is via standard stdio, where fbcon then either scrolls the screen via redrawing characters at new positions or by using hardware bitblt to move screen contents. IMHO there is nothing which is critical here. Even when I analyzed the existing bug reports, there was none which affected that specific code. Anyway, let's look at that part in your patch: + bool "Enable legacy fbcon code for hw acceleration" + depends on FRAMEBUFFER_CONSOLE + default n + help +Only enable this options if you are in full control of machine since +the fbcon acceleration code is essentially unmaintained and known +problematic. + +If unsure, select n. Since I'm willing to maintain that scrolling/panning code, I'd like to drop the "is essentially unmaintained" part. And the "known problematic" part is up to now just speculative (which would be valid for other parts of the kernel too, btw). As this whole disussions showed, there are some few architectures/platforms which really still need this acceleration, while others don't. So, why not leave the decision up to the arch maintainers, which may opt-in or opt-out, while keeping the default on "n"? With that, here is a new proposal: +config FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION + bool "Enable legacy fbcon hardware acceleration code" + depends on FRAMEBUFFER_CONSOLE + default y if (PARISC) # a few other arches may want to be listed here too... + default n + help + This o
Re: [PATCH v3 3/3] fbcon: Add option to enable legacy hardware acceleration
On Tue, Feb 1, 2022 at 7:59 PM Helge Deller wrote:
>
> Add a config option CONFIG_FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION to
> enable bitblt and fillrect hardware acceleration in the framebuffer
> console. If disabled, such acceleration will not be used, even if it is
> supported by the graphics hardware driver.
>
> If you plan to use DRM as your main graphics output system, you should
> disable this option since it will prevent compiling in code which isn't
> used later on when DRM takes over.
>
> For all other configurations, e.g. if none of your graphic cards support
> DRM (yet), DRM isn't available for your architecture, or you can't be
> sure that the graphic card in the target system will support DRM, you
> most likely want to enable this option.
>
> In the non-accelerated case (e.g. when DRM is used), the inlined
> fb_scrollmode() function is hardcoded to return SCROLL_REDRAW and as such the
> compiler is able to optimize much unneccesary code away.
>
> In this v3 patch version I additionally changed the GETVYRES() and GETVXRES()
> macros to take a pointer to the fbcon_display struct. This fixes the build
> when
> console rotation is enabled and helps the compiler again to optimize out code.
>
> Signed-off-by: Helge Deller
> Cc: sta...@vger.kernel.org # v5.10+
> Signed-off-by: Helge Deller
> ---
> drivers/video/console/Kconfig | 11 +++
> drivers/video/fbdev/core/fbcon.c| 39 ++---
> drivers/video/fbdev/core/fbcon.h| 15 +-
> drivers/video/fbdev/core/fbcon_ccw.c| 10 +++
> drivers/video/fbdev/core/fbcon_cw.c | 10 +++
> drivers/video/fbdev/core/fbcon_rotate.h | 4 +--
> drivers/video/fbdev/core/fbcon_ud.c | 20 ++---
> 7 files changed, 75 insertions(+), 34 deletions(-)
>
> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
> index 840d9813b0bc..6029fd41d7d0 100644
> --- a/drivers/video/console/Kconfig
> +++ b/drivers/video/console/Kconfig
> @@ -78,6 +78,17 @@ config FRAMEBUFFER_CONSOLE
> help
> Low-level framebuffer-based console driver.
>
> +config FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION
> + bool "Framebuffer Console hardware acceleration support"
> + depends on FRAMEBUFFER_CONSOLE
> + default n if DRM
> + default y
> + help
> + If you use a system on which DRM is fully supported you usually
> want to say N,
> + otherwise you probably want to enable this option.
> + If enabled the framebuffer console will utilize the hardware
> acceleration
> + of your graphics card by using hardware bitblt and fillrect
> features.
This really doesn't have much to do with DRM but more about running
questionable code. That's why I went with the generalized stern
warning and default n, and explained when it's ok to do this (single
user and you care more about fbcon performance than potential issues
because you only run trusted stuff with access to your vt and fbdev
/dev node). Also you didn't keep any todo entry for removing DRM accel
code, and iirc some nouveau folks also complained that they at least
once had some kind of accel, so that's another reason to not tie this
to DRM.
Anyway aside from this looks reasonable, can you pls respin with a
more cautious Kconfig text?
Thanks, Daniel
> +
> config FRAMEBUFFER_CONSOLE_DETECT_PRIMARY
> bool "Map the console to the primary display device"
> depends on FRAMEBUFFER_CONSOLE
> diff --git a/drivers/video/fbdev/core/fbcon.c
> b/drivers/video/fbdev/core/fbcon.c
> index b813985f1403..f7b7d35953e8 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -1136,11 +1136,13 @@ static void fbcon_init(struct vc_data *vc, int init)
>
> ops->graphics = 0;
>
> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION
> if ((cap & FBINFO_HWACCEL_COPYAREA) &&
> !(cap & FBINFO_HWACCEL_DISABLED))
> p->scrollmode = SCROLL_MOVE;
> else /* default to something safe */
> p->scrollmode = SCROLL_REDRAW;
> +#endif
>
> /*
> * ++guenther: console.c:vc_allocate() relies on initializing
> @@ -1705,7 +1707,7 @@ static bool fbcon_scroll(struct vc_data *vc, unsigned
> int t, unsigned int b,
> count = vc->vc_rows;
> if (logo_shown >= 0)
> goto redraw_up;
> - switch (p->scrollmode) {
> + switch (fb_scrollmode(p)) {
> case SCROLL_MOVE:
> fbcon_redraw_blit(vc, info, p, t, b - t - count,
> count);
> @@ -1795,7 +1797,7 @@ static bool fbcon_scroll(struct vc_data *vc, unsigned
> int t, unsigned int b,
> count = vc->vc_rows;
> if (logo_shown >= 0)
> goto redraw_down;
> - switch (p->scrollmode) {
> + switch (fb_scrollmo
