On 03/09/2021 09:51, Boris Brezillon wrote:

On Thu, 2 Sep 2021 15:00:38 +0100 Steven Price steven.price@arm.com wrote:

...

It turns out that when locking a region, the region must be a naturally aligned power of 2. The upshot of this is that if the desired region crosses a 'large boundary' the region size must be increased significantly to ensure that the locked region completely covers the desired region. Previous calculations (including in kbase for the proprietary driver) failed to take this into account.

Since it's known that the lock region must be naturally aligned we can compute the required size by looking at the highest bit position which changes between the start/end of the lock region (subtracting 1 from the end because the end address is exclusive). The start address is then aligned based on the size (this is technically unnecessary as the hardware will ignore these bits, but the spec advises to do this "to avoid confusion").

Signed-off-by: Steven Price steven.price@arm.com

See previous discussion[1] for more details. This bug also existed in the 'kbase' driver, so it's unlikely to actually hit very often.

This patch is based on drm-misc-next-fixes as it builds on top of Alyssa's changes to lock_region.

[1] https://lore.kernel.org/dri-devel/6fe675c4-d22b-22da-ba3c-f6d33419b9ed@arm.c...

drivers/gpu/drm/panfrost/panfrost_mmu.c | 33 +++++++++++++++++++------ 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c index dfe5f1d29763..afec15bb3db5 100644 --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c @@ -58,17 +58,36 @@ static int write_cmd(struct panfrost_device *pfdev, u32 as_nr, u32 cmd) }

static void lock_region(struct panfrost_device *pfdev, u32 as_nr,

• 	u64 iova, u64 size)
  

• 	u64 region_start, u64 size)
  

{ u8 region_width;

• u64 region = iova & PAGE_MASK;

• u64 region;
• u64 region_size;
• u64 region_end = region_start + size;

• /* The size is encoded as ceil(log2) minus(1), which may be calculated

• * with fls. The size must be clamped to hardware bounds.
  

• if (!size)

• return;
  

• /*

• * The locked region is a naturally aligned power of 2 block encoded as
  

• * log2 minus(1).
  

• * Calculate the desired start/end and look for the highest bit which
  

• * differs. The smallest naturally aligned block must include this bit
  

• * change the desired region starts with this bit (and subsequent bits)
  

• * zeroed and ends with the bit (and subsequent bits) set to one.
  

• *
  

Nit: you can drop the empty comment line.

Whoops - I reordered this comment and didn't spot the blank line getting left.

...

*/

• size = max_t(u64, size, AS_LOCK_REGION_MIN_SIZE);
• region_width = fls64(size - 1) - 1;
• region |= region_width;

• region_size = region_start ^ (region_end - 1);

Hm, is region_size really encoding the size of the region to lock? I mean, the logic seems correct but I wonder if it wouldn't be better to drop the region_size variable and inline 'region_start ^ (region_end - 1)' in the region_width calculation to avoid confusion.

Yeah I wasn't happy about the variable name either, but I couldn't think of a better one. Inlining it into the following line nicely avoids the problem ;)

Looks good otherwise.

Reviewed-by: Boris Brezillon boris.brezillon@collabora.com

Thanks, I'll post a v2 in case anyone else has other comments.

Steve

...

• region_width = max(fls64(region_size),

• 	   const_ilog2(AS_LOCK_REGION_MIN_SIZE)) - 1;
  

• /*

• * Mask off the low bits of region_start (which would be ignored by
  

• * the hardware anyway)
  

• */
  

• region_start &= GENMASK_ULL(63, region_width);

• region = region_width | region_start;

  /* Lock the region that needs to be updated */ mmu_write(pfdev, AS_LOCKADDR_LO(as_nr), region & 0xFFFFFFFFUL);