linux-kernel - Re: [PATCH 1/7] ttm/radeon/nouveau: Check the DMA address from TTM against known value.

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Date:	Wed, 31 Aug 2011 08:02:09 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Pekka Paalanen <pq@....fi>
Cc:	thellstrom@...are.com, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, bskeggs@...hat.com,
	j.glisse@...hat.com, thomas@...pmail.org, airlied@...hat.com,
	airlied@...ux.ie, alexdeucher@...il.com
Subject: Re: [PATCH 1/7] ttm/radeon/nouveau: Check the DMA address from TTM
 against known value.

On Wed, Aug 31, 2011 at 09:33:29AM +0300, Pekka Paalanen wrote:
> On Tue, 30 Aug 2011 22:41:46 -0400
> Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> 
> > . instead of checking against the DMA_ERROR_CODE value which is
> > per-platform specific. The zero value is a known invalid value
> > that the TTM layer sets on the dma_address array if it is not
> > used (ttm_tt_alloc_page_directory calls drm_calloc_large which
> > creates a page with GFP_ZERO).
> > 
> > We can't use pci_dma_mapping_error as that is IOMMU
> > specific (some check for a specific physical address, some
> > for ranges, some just do a check against zero).
> > 
> > Also update the comments in the header about the true state
> > of that parameter.
> > 
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > ---
> >  drivers/gpu/drm/nouveau/nouveau_sgdma.c |    3 +--
> >  drivers/gpu/drm/radeon/radeon_gart.c    |    4 +---
> >  include/drm/ttm/ttm_page_alloc.h        |    4 ++--
> >  3 files changed, 4 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_sgdma.c
> > b/drivers/gpu/drm/nouveau/nouveau_sgdma.c index 82fad91..624e2db
> > 100644 --- a/drivers/gpu/drm/nouveau/nouveau_sgdma.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_sgdma.c
> > @@ -42,8 +42,7 @@ nouveau_sgdma_populate(struct ttm_backend *be,
> > unsigned long num_pages, 
> >  	nvbe->nr_pages = 0;
> >  	while (num_pages--) {
> > -		/* this code path isn't called and is incorrect
> > anyways */
> > -		if (0) { /*dma_addrs[nvbe->nr_pages] !=
> > DMA_ERROR_CODE)*/
> > +		if (dev->pdev, dma_addrs[nvbe->nr_pages] != 0) {
> 
> This is weird, do you mean && instead of a comma, or what?
> Or am I completely missing the comma operator semantics?

Earlier implementation had this:

if (!pci_dma_mapping_error(rdev->pdev, dma_addr[i])) {

And then I changed it to check just the dma_addrs[x] (as the
different IOMMUs would provide irregular values), but
this ',' is really weird - no idea how it actually even compiles.

It should have just been:

		if (dma_addrs[nvbe->nr_pages] != 0) {
> 
> >  			nvbe->pages[nvbe->nr_pages] =
> >  					dma_addrs[nvbe->nr_pages];
> >  		 	nvbe->ttm_alloced[nvbe->nr_pages] =
> > true; diff --git a/drivers/gpu/drm/radeon/radeon_gart.c
> > b/drivers/gpu/drm/radeon/radeon_gart.c index a533f52..41f7e51
> > 100644 --- a/drivers/gpu/drm/radeon/radeon_gart.c
> > +++ b/drivers/gpu/drm/radeon/radeon_gart.c
> > @@ -181,9 +181,7 @@ int radeon_gart_bind(struct radeon_device
> > *rdev, unsigned offset, p = t / (PAGE_SIZE /
> > RADEON_GPU_PAGE_SIZE); 
> >  	for (i = 0; i < pages; i++, p++) {
> > -		/* we reverted the patch using dma_addr in TTM
> > for now but this
> > -		 * code stops building on alpha so just comment
> > it out for now */
> > -		if (0) { /*dma_addr[i] != DMA_ERROR_CODE) */
> > +		if (rdev->pdev, dma_addr[i] != 0) {
> 
> The same question for this condition.

The same here. Reading
http://stackoverflow.com/questions/2087026/effect-of-using-a-comma-instead-of-a-semi-colon-in-c-and-c

says that it actually did the right thing (evaluated the last
thing) - but I am going to remove the pdev part.

Thanks for spotting this!
> 
> -- 
> Pekka Paalanen
> http://www.iki.fi/pq/
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