[Intel-gfx] [RFC PATCH 39/97] drm/i915/guc: Increase size of CTB buffers

[Michal Wajdeczko]

On 06.05.2021 21:13, Matthew Brost wrote:
> With the introduction of non-blocking CTBs more than one CTB can be in
> flight at a time. Increasing the size of the CTBs should reduce how
> often software hits the case where no space is available in the CTB
> buffer.
> 
> Cc: John Harrison <john.c.harrison at intel.com>
> Signed-off-by: Matthew Brost <matthew.brost at intel.com>
> ---
>  drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> index 77dfbc94dcc3..d6895d29ed2d 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> @@ -63,11 +63,16 @@ static inline struct drm_device *ct_to_drm(struct intel_guc_ct *ct)
>   *      +--------+-----------------------------------------------+------+
>   *
>   * Size of each `CT Buffer`_ must be multiple of 4K.
> - * As we don't expect too many messages, for now use minimum sizes.
> + * We don't expect too many messages in flight at any time, unless we are
> + * using the GuC submission. In that case each request requires a minimum
> + * 16 bytes which gives us a maximum 256 queue'd requests. Hopefully this

nit: all our CTB calculations are in dwords now, not bytes

> + * enough space to avoid backpressure on the driver. We increase the size
> + * of the receive buffer (relative to the send) to ensure a G2H response
> + * CTB has a landing spot.

hmm, but we are not checking G2H CTB yet
will start doing it around patch 54/97
so maybe this other patch should be introduced earlier ?

>   */
>  #define CTB_DESC_SIZE		ALIGN(sizeof(struct guc_ct_buffer_desc), SZ_2K)
>  #define CTB_H2G_BUFFER_SIZE	(SZ_4K)
> -#define CTB_G2H_BUFFER_SIZE	(SZ_4K)
> +#define CTB_G2H_BUFFER_SIZE	(4 * CTB_H2G_BUFFER_SIZE)

in theory, we (host) should be faster than GuC, so G2H CTB shall be
almost always empty, if this is not a case, maybe we should start
monitoring what is happening and report some warnings if G2H is half full ?

>  
>  #define MAX_US_STALL_CTB	1000000
>  
> @@ -753,7 +758,7 @@ static int ct_read(struct intel_guc_ct *ct, struct ct_incoming_msg **msg)
>  	/* beware of buffer wrap case */
>  	if (unlikely(available < 0))
>  		available += size;
> -	CT_DEBUG(ct, "available %d (%u:%u)\n", available, head, tail);
> +	CT_DEBUG(ct, "available %d (%u:%u:%u)\n", available, head, tail, size);
>  	GEM_BUG_ON(available < 0);
>  
>  	header = cmds[head];
>