On 03/09/2021 10:49, Steven Price 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").
>
> Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> Signed-off-by: Steven Price <steven.price@xxxxxxx>
Pushed to drm-misc-next, thanks for the reviews.
Steve
> ---
> drivers/gpu/drm/panfrost/panfrost_mmu.c | 30 +++++++++++++++++++------
> 1 file changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index dfe5f1d29763..e2629b8d6a02 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -58,17 +58,33 @@ 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_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.
> */
> - size = max_t(u64, size, AS_LOCK_REGION_MIN_SIZE);
> - region_width = fls64(size - 1) - 1;
> - region |= region_width;
> + region_width = max(fls64(region_start ^ (region_end - 1)),
> + 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);
>
