On 2/28/24 11:19, Thomas Zimmermann wrote: > Hi > > Am 27.02.24 um 19:14 schrieb Dmitry Osipenko: >> Hello, >> >> Thank you for the patches! >> >> On 2/27/24 13:14, Thomas Zimmermann wrote: >>> Dma-buf locking semantics require the caller of pin and unpin to hold >>> the buffer's reservation lock. Fix DRM to adhere to the specs. This >>> enables to fix the locking in DRM's console emulation. Similar changes >>> for vmap and mmap have been posted at [1][2] >>> >>> Most DRM drivers and memory managers acquire the buffer object's >>> reservation lock within their GEM pin and unpin callbacks. This >>> violates dma-buf locking semantics. We get away with it because PRIME >>> does not provide pin/unpin, but attach/detach, for which the locking >>> semantics is correct. >>> >>> Patches 1 to 8 rework DRM GEM code in various implementations to >>> acquire the reservation lock when entering the pin and unpin callbacks. >>> This prepares them for the next patch. Drivers that are not affected >>> by these patches either don't acquire the reservation lock (amdgpu) >>> or don't need preparation (loongson). >>> >>> Patch 9 moves reservation locking from the GEM pin/unpin callbacks >>> into drm_gem_pin() and drm_gem_unpin(). As PRIME uses these functions >>> internally it still gets the reservation lock. >>> >>> With the updated GEM callbacks, the rest of the patchset fixes the >>> fbdev emulation's buffer locking. Fbdev emulation needs to keep its >>> GEM buffer object inplace while updating its content. This required >>> a implicit pinning and apparently amdgpu didn't do this at all. >>> >>> Patch 10 introduces drm_client_buffer_vmap_local() and _vunmap_local(). >>> The former function map a GEM buffer into the kernel's address space >>> with regular vmap operations, but keeps holding the reservation lock. >>> The _vunmap_local() helper undoes the vmap and releases the lock. The >>> updated GEM callbacks make this possible. Between the two calls, the >>> fbdev emulation can update the buffer content without have the buffer >>> moved or evicted. Update fbdev-generic to use vmap_local helpers, >>> which fix amdgpu. The idea of adding a "local vmap" has previously been >>> attempted at [3] in a different form. >>> >>> Patch 11 adds implicit pinning to the DRM client's regular vmap >>> helper so that long-term vmap'ed buffers won't be evicted. This only >>> affects fbdev-dma, but GEM DMA helpers don't require pinning. So >>> there are no practical changes. >>> >>> Patches 12 and 13 remove implicit pinning from the vmap and vunmap >>> operations in gem-vram and qxl. These pin operations are not supposed >>> to be part of vmap code, but were required to keep the buffers in place >>> for fbdev emulation. With the conversion o ffbdev-generic to to >>> vmap_local helpers, that code can finally be removed. >> Isn't it a common behaviour for all DRM drivers to implicitly pin BO >> while it's vmapped? I was sure it should be common /o\ > > That's what I originally thought as well, but the intention is for pin > and vmap to be distinct operation. So far each driver has been > different, as you probably know best from your vmap refactoring. :) > >> >> Why would you want to kmap BO that isn't pinned? > > Pinning places the buffer object for the GPU. As a side effect, the > buffer is then kept in place, which enables vmap. So pinning only makes > sense for buffer objects that never move (shmem, dma). That's what patch > 11 is for. > >> >> Shouldn't TTM's vmap() be changed to do the pinning? > > I don't think so. One problem is that pinning needs a memory area (vram, > GTT, system ram, etc) specified, which vmap simply doesn't know about. > That has been a problem for fbdev emulation at some point. Our fbdev > code tried to pin as part of vmap, but chose the wrong area and suddenly > the GPU could not see the buffer object any longer. So the next best > thing for vmap was to pin the buffer object where ever it is currently > located. That is what gem-vram and qxl did so far. And of course, the > fbdev code needs to unpin and vunmap the buffer object quickly, so that > it can be relocated if the GPU needs it. Hence, the vmap_local > interface removes such short-term pinning in favor of holding the > reservation lock. > >> >> I missed that TTM doesn't pin BO on vmap() and now surprised to see it. >> It should be a rather serious problem requiring backporting of the >> fixes, but I don't see the fixes tags on the patches (?) > > No chance TBH. The old code has worked for years and backporting all > this would require your vmap patches at a minimum. > > Except maybe for amdgpu. It uses fbdev-generic, which requires pinning, > but amdgpu doesn't pin. That looks fishy, but I'm not aware of any bug > reports either. I guess, a quick workaround could fix older amdgpu if > necessary. Thanks! I'll make another pass on the patches on Monday -- Best regards, Dmitry
