On Wed, Nov 01, 2023 at 12:43:25PM +0900, Tomasz Figa wrote:
On Tue, May 16, 2023 at 7:50 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:Hi Michael, On Wed, May 10, 2023 at 11:25 PM Michael Grzeschik <mgr@xxxxxxxxxxxxxx> wrote: > > Sorry for the late comeback, however here are some thoughts. > > On Fri, Dec 02, 2022 at 06:01:02PM +0900, Tomasz Figa wrote: > >On Thu, Nov 24, 2022 at 10:35 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote: > >> > >> On 21/11/2022 00:44, Michael Grzeschik wrote: > >> > The comments before the vm_map_ram function state that it should be used > >> > for up to 256 KB only, and video buffers are definitely much larger. It > >> > recommends using vmap in that case. > >> > > >> > Signed-off-by: Michael Grzeschik <m.grzeschik@xxxxxxxxxxxxxx> > >> > --- > >> > drivers/media/common/videobuf2/videobuf2-dma-sg.c | 7 ++++--- > >> > >> drivers/media/common/videobuf2/videobuf2-vmalloc.c uses it as well, > >> probably also incorrectly. It makes sense to change that one as well. > > > >Comparing vm_map_ram() and vmap(..., VM_MAP, PAGE_KERNEL), for blocks > >bigger than VMAP_MAX_ALLOC they're equivalent and for smaller blocks > >the former should be faster, so I don't see what's wrong with the > >current code. > > I got another comment on this from Andrzej Pietrasiewicz > where he expands the comment on the use of vmap over vm_map_ram. > > https://lore.kernel.org/linux-media/64375ff4-dbbb-3d5b-eaf6-32d6780fd496@xxxxxxxxxxxxx > > As I understand this, we should probably update the vm_map_ram to vmap, > due to the expectation that video buffers are long-living objects. > > Since there are some more places that would probably need to be updated > if we should decide to use vmap over vm_map_ram in the whole > videbuf2-* users, I would like to clarify on this before making > a series. Ah, I see. Thanks for the pointer. VB2 buffers would usually require long-lived mappings, so based on that, we should switch to vmap() indeed. As a side note, not directly related to this patch, I wonder if we should also call invalidate/flush_kernel_vmap_range() in vb2_dma_sg_prepare/finish(). (In principle we shouldn't, but so far our drivers don't explicitly call begin/end_cpu_access() and rely on prepare/finish to handle the cache maintenance of the kernel mapping...) Let me also add Sergey on CC for visibility.Sorry, I forgot last time, so maybe it wasn't clear I'm good with this patch: Acked-by: Tomasz Figa <tfiga@xxxxxxxxxxxx> Hans, will you pick it? Thanks!
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