Re: [PATCH v10 07/11] drm/etnaviv: Add support for the dma coherent device

[Sui Jingfeng]

On 2023/6/21 23:23, Lucas Stach wrote:
> Am Mittwoch, dem 21.06.2023 um 22:44 +0800 schrieb Sui Jingfeng:
> > Hi,
> >
> > On 2023/6/21 18:00, Lucas Stach wrote:
> > > > diff --git a/drivers/gpu/drm/etnaviv/etnaviv_drv.h b/drivers/gpu/drm/etnaviv/etnaviv_drv.h
> > > > index 9cd72948cfad..644e5712c050 100644
> > > > --- a/drivers/gpu/drm/etnaviv/etnaviv_drv.h
> > > > +++ b/drivers/gpu/drm/etnaviv/etnaviv_drv.h
> > > > @@ -46,6 +46,12 @@ struct etnaviv_drm_private {
> > > >    	struct xarray active_contexts;
> > > >    	u32 next_context_id;
> > > >    
> > > > +	/*
> > > > +	 * If true, the GPU is capable of snooping cpu cache. Here, it
> > > > +	 * also means that cache coherency is enforced by the hardware.
> > > > +	 */
> > > > +	bool dma_coherent;
> > > > +
> > > No need for this, I think. Just use dev_is_dma_coherent() where you
> > > need to know this.
> > No, we want this value cached by the driver.
> Why? dev_is_dma_coherent() is a header-only function with a single
> pointer chasing operation. Your cache is also a single pointer chasing
> access, just that we now need storage for this information in both
> struct device and struct etnaviv_gpu.


You don't need store it in struct etnaviv_gpu.

As this variable is shared across the device, so it is better to be put 
in the struct etnaviv_drm_private.

I don't think another 4 bytes allocation is something what we can't pay for.


My patch doesn't mentioned that it need to store it inside of struct 
etnaviv_gpu, do I?

> Regards,
> Lucas
>
> > We only need call  dev_is_dma_coherent() once!
> >
> > We need to reuse this variable on other places.
--
Jingfeng