Hi Stephen On Fri, 26 Jul 2013, Stephen Warren wrote:
> On 07/26/2013 02:23 PM, Guennadi Liakhovetski wrote: > > Hi Stephen > > > > On Fri, 26 Jul 2013, Stephen Warren wrote: > > > >> On 07/26/2013 09:51 AM, Guennadi Liakhovetski wrote: > >>> Add DT properties for UHS SDR12, SDR25, SDR50, SDR104 and DDR50 modes and > >>> for supported by the host in DDR mode VccQ values. Adding them to DT will > >>> automatically enable respective MMC host capabilities. > >> > >>> diff --git a/Documentation/devicetree/bindings/mmc/mmc.txt > >>> b/Documentation/devicetree/bindings/mmc/mmc.txt > >> > >>> +- uhs-sdr12: the host supports UHS SDR12 mode > >>> +- uhs-sdr25: the host supports UHS SDR25 mode > >>> +- uhs-sdr50: the host supports UHS SDR50 mode > >>> +- uhs-sdr104: the host supports UHS SDR104 mode > >>> +- uhs-ddr50: the host supports UHS DDR50 mode > >>> +- ddr-1v2: the host can support DDR, using 1.2V VccQ > >>> +- ddr-1v8: the host can support DDR, using 1.8V VccQ > >> > >> Surely the driver for the host controller already knows this, so there's > >> no need to represent it in DT? > > > > Not necessarily. One driver can handle several versions of the same chip, > > and some versions of the chip implement some of those features, others > > don't. > > Certainly. > > > So, your choice is really whether to specify a chip version in the > > compatible string or these properties. > > There's no choice there. The compatible property needs to specify all of > the following: > > * A value which describes the exact chip the IP block is in. This can be > matched on to enable any quirks that need to be handled due to > integration of the IP into the particular chip. This value will list the > chip version. An example might be tegra20-sdhci. > > * A value which describes the IP block version (if that IP block has a > version independent of the SoC that contains it, as e.g. a Synopsis IP > block might). A totally made-up example might be synopsis-dwc2-1.0.0 > > * Various more generic values that describe the HW, and that define a n > interface to the HW that is specific and complete enough that HW can > program to. An example might be usb-ehci (assuming a chip that doesn't > have so many quirks that a driver has to know more than just "it's an > EHCI controller). Yes, all these certainly make sense. As far as I understand, we are still in the process of defining good clear rules for DTs, there is an "ABI" discussion currently running on ALKML and IIRC this is also going to be a topic for one of coming conferences. So, hopefully we're approaching a state of a greater clarity about DT. > Further classes of compatible value might exist, but you get the idea. > > All of those values have to exist in the DT right from the very start, > so that the first DT is usable with a future kernel, which might decide > to vary the exact compatible value(s) it matches on, provided they're > all documented in the DT binding ABI, in order to enable/disable new > sets of quirsk. Makes sense too, sure. > > Now, when you consider that > > multiple drivers have to decide upon those, and sometimes you don't have > > an exact IP version of the SD/MMC controller but only the SoC version, you > > choice becomes: > > That would be a bug in the DT, given my assertions above. Sorry, what exactly would be a bug? Not having an exact IP version? But you did write above - "if that IP block has a version independent of the SoC that contains it." In my case it doesn't really. Those IPs only exist within those SoCs. > > either _standard_ _common_ properties as above, or > > compatibility strings virtually in each driver for each SoC version. > > My preference would certainly be to derive the data from compatible > strings here. I'd prefer more that DT represent board differences rather > than SoC differences; drivers can encompass the SoC differences with > minimal code in general. In general - yes, I fully agree. I wasn't sure about this specific case, exactly because those are standard properties, that, if implemented as DT properties, can be trivially re-used by all drivers with 0 additional code. > Related, is it really true that zero driver involvement is required to > enable these UHS features? I think it is, in most cases at least. > If absolutely any HW can enable them without > any driver support at all, then perhaps it's still reasonable to create > DT properties to enable them. However, if driver support is required to > make those features actually work, the driver had better be validating > that support actually works on the HW it's running on before enabling > the feature (and can therefore pass the validation results to the SD > core rather than relying on DT properties to be set). I think it rather works the other way round. Those flags only mark hardware _capabilities_. But don't tell the driver to use them yet. Normally drivers just collect them in their .caps and .caps2 fields and pass on to the mmc core. The mmc core then card detection and querying and if a card and the host interface to it are suitable for certain features, marked in those capability fields, the core will then ask the host driver to enable it. Of course, I don't know all drivers some drivers might well behave differently, but even then they anyway are the first to find out about those capabilities. After calling mmc_of_parse() drivers get those capabilities back and can do whatever needs to be done to validate them, and can even clear them again, if they find out, that some of them aren't really available. mmc_of_parse() just makes driver's work a bit easier, but doesn't take control away from it. > I'm pretty sure > that our downstream Tegra SDHCI driver contains a lot of code to make > UHS actually work, even though the HW does support it, and hence with > this patch applied, the DT would request that it be enabled. > > > That's why I decided to use explicit properties for those. Example: > > sh_mmcif driver supports MMCIF IP blocks on various Renesas sh-, r-mobile > > and r-car SuperH and ARM SoCs. On some of them DDR50 is supported, on > > others it isn't. Thanks Guennadi --- Guennadi Liakhovetski, Ph.D. Freelance Open-Source Software Developer http://www.open-technology.de/ -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
