[PATCH] drm/ttm: Use ttm_bo_default_io_mem_pfn if io_mem_pfn is NULL

[Sean Paul]

On Wed, Nov 29, 2017 at 04:59:29PM +0100, Christian König wrote:
> Am 29.11.2017 um 16:44 schrieb Sean Paul:
> > On Wed, Nov 29, 2017 at 04:40:18PM +0100, Christian König wrote:
> > > Am 29.11.2017 um 16:36 schrieb Sean Paul:
> > > > On Wed, Nov 29, 2017 at 04:24:21PM +0100, Christian König wrote:
> > > > > Am 29.11.2017 um 16:20 schrieb Michal Srb:
> > > > > > The io_mem_pfn field was added in commit ea642c3216cb2a60d1c0e760ae47ee85c9c16447
> > > > > > and is called unconditionally. However, not all drivers were updated to set it.
> > > > > > 
> > > > > > Use the ttm_bo_default_io_mem_pfn function if a driver did not set its own.
> > > > > > 
> > > > > > Signed-off-by: Michal Srb <msrb at suse.com>
> > > > > NAK, when we have drivers missing this we should set this in the driver and
> > > > > not add the workaround here.
> > > > Why? What's the benefit in adding the same hook in 10 drivers (I was hoping
> > > > Michal would also remove the token .io_mem_pfn = ttm_bo_default_io_mem_pfn from
> > > > those as well)?
> > > Not have this extra check in common TTM code? That is seriously bad coding
> > > style.
> > > 
> > [citation neeeded]
> > 
> > I'd argue it's bad form to require everyone add a copypasta hook to their driver
> > everytime an unrelated driver needs more control than the default gives :-)
> 
> Well that is another design problem of TTM that it works like a middle layer
> instead of a tool. But I'm seriously not going to fix this anytime soon.
> 
> > I think your point would hold if we were talking about a couple of drivers, but
> > once you scale to 10+, it's no longer a trivial amount of code. If the null check
> > is too distracting, you could always pull it out into an inline.
> Yeah, that is one possibility that would work for me as well.
> 
> TTM has patterns like the following:
> 
> First occasion:
> if (a_is_available)
>     do_a();
> else
>     do_b();
> 
> Same code a bit lower:
> if (a_is_available)
>     do_a()
> /* Else is missing here */
> 
> I'm seriously favoring updating a dozen drivers just to avoid more of this.
> 
> If you want to fix that with adding wrapper functions to call the driver
> callback and give a consistent operation when the callback isn't available
> then I'm perfectly fine with that as well. Patches are always welcome.

Yeah, that's what I was asking Michal for. I'd like a patch series where:

[0/N] adds the inline to call the default if io_mem_pfn is NULL
[1..N-1/N] remove all .io_mem_pfn = ttm_bo_default_io_mem_pfn from drivers
[N/N] unexport ttm_bo_default_io_mem_pfn and make it static

Sean


> 
> Christian.
> 
> > 
> > Sean
> > 
> > 
> > > We have tons of fallbacks like this and I unfortunately even created new one
> > > without realizing that this is a problem.
> > > 
> > > But this really clutters the TTM code and makes it harder to understand.
> > > 
> > > Christian.
> > > 
> > > > Sean
> > > > 
> > > > > Which driver is missing this? I thought I fixed all of them.
> > > > > 
> > > > > Regards,
> > > > > Christian.
> > > > > 
> > > > > > ---
> > > > > >     drivers/gpu/drm/ttm/ttm_bo_vm.c | 5 ++++-
> > > > > >     1 file changed, 4 insertions(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > > > > index c8ebb757e36b..e25a99bc519d 100644
> > > > > > --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > > > > +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > > > > @@ -234,7 +234,10 @@ static int ttm_bo_vm_fault(struct vm_fault *vmf)
> > > > > >     		if (bo->mem.bus.is_iomem) {
> > > > > >     			/* Iomem should not be marked encrypted */
> > > > > >     			cvma.vm_page_prot = pgprot_decrypted(cvma.vm_page_prot);
> > > > > > -			pfn = bdev->driver->io_mem_pfn(bo, page_offset);
> > > > > > +			if (bdev->driver->io_mem_pfn)
> > > > > > +				pfn = bdev->driver->io_mem_pfn(bo, page_offset);
> > > > > > +			else
> > > > > > +				pfn = ttm_bo_default_io_mem_pfn(bo, page_offset);
> > > > > >     		} else {
> > > > > >     			page = ttm->pages[page_offset];
> > > > > >     			if (unlikely(!page && i == 0)) {
> > > > > _______________________________________________
> > > > > dri-devel mailing list
> > > > > dri-devel at lists.freedesktop.org
> > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS