Subject: Re: [PATCH] firmware: arm_ffa: Use the correct buffer size during RXTX_MAP

From: Sebastian Ene <[email-masked]>
Date: Wed, 1 Apr 2026 17:09:49 +0000

On Wed, Apr 01, 2026 at 05:21:02PM +0100, Sudeep Holla wrote:
> On Wed, Apr 01, 2026 at 11:17:38AM +0000, Sebastian Ene wrote:
> > Don't use the discovered buffer size from an FFA_FEATURES call directly
> > since we can run on a system that has the PAGE_SIZE larger than the
> > returned size which makes the alloc_pages_exact for the buffer to be
> > rounded up.
> > 
> > Fixes: 61824feae5c0 ("firmware: arm_ffa: Fetch the Rx/Tx buffer size using ffa_features()")
> > Signed-off-by: Sebastian Ene <[email-masked]>
> > ---
> >  drivers/firmware/arm_ffa/driver.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/firmware/arm_ffa/driver.c b/drivers/firmware/arm_ffa/driver.c
> > index f2f94d4d533e..d0c926aca120 100644
> > --- a/drivers/firmware/arm_ffa/driver.c
> > +++ b/drivers/firmware/arm_ffa/driver.c
> > @@ -2078,7 +2078,7 @@ static int __init ffa_init(void)
> >  
> >  	ret = ffa_rxtx_map(virt_to_phys(drv_info->tx_buffer),
> >  			   virt_to_phys(drv_info->rx_buffer),
> > -			   rxtx_bufsz / FFA_PAGE_SIZE);
> > +			   DIV_ROUND_UP(rxtx_bufsz, PAGE_SIZE) / FFA_PAGE_SIZE);
> 
> Did you mean to use PAGE_ALIGN() instead of DIV_ROUND_UP() ?

Yes, PAGE_ALIGN() my bad, I will send a v2 for this.

> 
> The new pg_cnt calculation is dimensionally incorrect and evaluates to 0 for
> all the supported buffer sizes, so FFA_RXTX_MAP is always called with an
> invalid page count.
> 
> Example: with rxtx_bufsz = 4K, the expression becomes 1 / 4096 on 4K/16K/64K
> page kernel, so pg_cnt == 0. Wouldn’t this  cause a boot-time regression for
> FF-A driver init ? Have you tested this ? I am trying to understand what I
> might be missing here.
> 

Thanks,
Sebastian

> -- 
> Regards,
> Sudeep