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Message-ID: <87c9576e6ca1b58fa94e0bc1a2f4be3847f0518c.camel@pengutronix.de>
Subject: Re: [PATCH v10 07/11] drm/etnaviv: Add support for the dma coherent
 device
From:   Lucas Stach <l.stach@pengutronix.de>
To:     Sui Jingfeng <suijingfeng@loongson.cn>,
        Sui Jingfeng <18949883232@163.com>,
        Russell King <linux+etnaviv@armlinux.org.uk>,
        Christian Gmeiner <christian.gmeiner@gmail.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>
Cc:     linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        etnaviv@lists.freedesktop.org,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Bjorn Helgaas <bhelgaas@google.com>
Date:   Wed, 21 Jun 2023 17:23:21 +0200
In-Reply-To: <aa73348d-5ec8-4ac0-2ec0-0cce24756c63@loongson.cn>
References: <20230620094716.2231414-1-18949883232@163.com>
         <20230620094716.2231414-8-18949883232@163.com>
         <8f74f0962c8bab6c832919a5340667c54e1a7ddc.camel@pengutronix.de>
         <aa73348d-5ec8-4ac0-2ec0-0cce24756c63@loongson.cn>
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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.

Regards,
Lucas

> We only need call  dev_is_dma_coherent() once!
> 
> We need to reuse this variable on other places.
>