Hi Christian and Thomas Am 16.09.20 um 15:37 schrieb Thomas Hellström (Intel): > > On 9/16/20 2:59 PM, Christian König wrote: >> Am 16.09.20 um 14:24 schrieb Daniel Vetter: >>> On Wed, Sep 16, 2020 at 12:48:20PM +0200, Thomas Zimmermann wrote: >>>> Hi >>>> >>>> Am 16.09.20 um 11:37 schrieb Daniel Vetter: >>>>> On Mon, Sep 14, 2020 at 01:25:18PM +0200, Thomas Zimmermann wrote: >>>>>> Dma-buf provides vmap() and vunmap() for retrieving and releasing >>>>>> mappings >>>>>> of dma-buf memory in kernel address space. The functions operate >>>>>> with plain >>>>>> addresses and the assumption is that the memory can be accessed >>>>>> with load >>>>>> and store operations. This is not the case on some architectures >>>>>> (e.g., >>>>>> sparc64) where I/O memory can only be accessed with dedicated >>>>>> instructions. >>>>>> >>>>>> This patchset introduces struct dma_buf_map, which contains the >>>>>> address of >>>>>> a buffer and a flag that tells whether system- or I/O-memory >>>>>> instructions >>>>>> are required. >>>>>> >>>>>> Some background: updating the DRM framebuffer console on sparc64 >>>>>> makes the >>>>>> kernel panic. This is because the framebuffer memory cannot be >>>>>> accessed with >>>>>> system-memory instructions. We currently employ a workaround in >>>>>> DRM to >>>>>> address this specific problem. [1] >>>>>> >>>>>> To resolve the problem, we'd like to address it at the most common >>>>>> point, >>>>>> which is the dma-buf framework. The dma-buf mapping ideally knows >>>>>> if I/O >>>>>> instructions are required and exports this information to it's >>>>>> users. The >>>>>> new structure struct dma_buf_map stores the buffer address and a >>>>>> flag that >>>>>> signals I/O memory. Affected users of the buffer (e.g., drivers, >>>>>> frameworks) >>>>>> can then access the memory accordingly. >>>>>> >>>>>> This patchset only introduces struct dma_buf_map, and updates >>>>>> struct dma_buf >>>>>> and it's interfaces. Further patches can update dma-buf users. For >>>>>> example, >>>>>> there's a prototype patchset for DRM that fixes the framebuffer >>>>>> problem. [2] >>>>>> >>>>>> Further work: TTM, one of DRM's memory managers, already exports an >>>>>> is_iomem flag of its own. It could later be switched over to >>>>>> exporting struct >>>>>> dma_buf_map, thus simplifying some code. Several DRM drivers >>>>>> expect their >>>>>> fbdev console to operate on I/O memory. These could possibly be >>>>>> switched over >>>>>> to the generic fbdev emulation, as soon as the generic code uses >>>>>> struct >>>>>> dma_buf_map. >>>>>> >>>>>> [1] >>>>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F20200725191012.GA434957%40ravnborg.org%2F&data=02%7C01%7Cchristian.koenig%40amd.com%7C04e3cc3e03ae40f1fa0f08d85a3b6a68%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637358558524732385&sdata=wTmFuB95GhKUU%2F2Q91V0%2BtzAu4%2BEe3VBUcriBy3jx2g%3D&reserved=0 >>>>>> >>>>>> [2] >>>>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F20200806085239.4606-1-tzimmermann%40suse.de%2F&data=02%7C01%7Cchristian.koenig%40amd.com%7C04e3cc3e03ae40f1fa0f08d85a3b6a68%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637358558524732385&sdata=L4rBHmegO63b%2FiTQdTyH158KNxAZwSuJCQOaFszo5L0%3D&reserved=0 >>>>>> >>>>> lgtm, imo ready to convert the follow-up patches over to this. But >>>>> I think >>>>> would be good to get at least some ack from the ttm side for the >>>>> overall >>>>> plan. >>>> Yup, it would be nice if TTM could had out these types automatically. >>>> Then all TTM-based drivers would automatically support it. >>>> >>>>> Also, I think we should put all the various helpers (writel/readl, >>>>> memset, >>>>> memcpy, whatever else) into the dma-buf-map.h helper, so that most >>>>> code >>>>> using this can just treat it as an abstract pointer type and never >>>>> look >>>>> underneath it. >>>> We have some framebuffer helpers that rely on pointer arithmetic, so >>>> we'd need that too. No big deal wrt code, but I was worried about the >>>> overhead. If a loop goes over framebuffer memory, there's an if/else >>>> branch for each access to the memory buffer. >>> If we make all the helpers static inline, then the compiler should be >>> able >>> to see that dma_buf_map.is_iomem is always the same, and produced really >>> optimized code for it by pulling that check out from all the loops. >>> >>> So should only result in somewhat verbose code of having to call >>> dma_buf_map pointer arthimetic helpers, but not in bad generated code. >>> Still worth double-checking I think, since e.g. on x86 the generated >>> code >>> should be the same for both cases (but maybe the compiler doesn't see >>> through the inline asm to realize that, so we might end up with 2 >>> copies). >> >> Can we have that even independent of DMA-buf? We have essentially the >> same problem in TTM and the code around that is a complete mess if you >> ask me. >> >> Christian. >> > I think this patchset looks good. Changing ttm_bo_kmap() over to > returning a struct dma-buf-map would probably work just fine. If we then > can have a set of helpers to operate on it, that's great. > > /Thomas
Can I count this as an A-b by one of you? Best regards Thomas > > > _______________________________________________ > dri-devel mailing list > dri-devel@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/dri-devel -- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Felix Imendörffer
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