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

[Sui Jingfeng]

Hi,

On 2023/6/22 00:07, Lucas Stach wrote:
> And as the HW guarantees it on your platform, your platform
> implementation makes this function effectively a no-op. Skipping the
> call to this function is breaking the DMA API abstraction, as now the
> driver is second guessing the DMA API implementation. I really see no
> reason to do this.

It is the same reason you chose the word 'effectively', not 'difinitely'.

We don't want waste the CPU's time,


 to running the dma_sync_sg_for_cpu funcion() function


```

void dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
            int nelems, enum dma_data_direction dir)
{
    const struct dma_map_ops *ops = get_dma_ops(dev);

    BUG_ON(!valid_dma_direction(dir));
    if (dma_map_direct(dev, ops))
        dma_direct_sync_sg_for_cpu(dev, sg, nelems, dir);
    else if (ops->sync_sg_for_cpu)
        ops->sync_sg_for_cpu(dev, sg, nelems, dir);
    debug_dma_sync_sg_for_cpu(dev, sg, nelems, dir);
}

```


 to running the this:


```

int etnaviv_gem_cpu_fini(struct drm_gem_object *obj)
{
    struct drm_device *dev = obj->dev;
    struct etnaviv_gem_object *etnaviv_obj = to_etnaviv_bo(obj);
    struct etnaviv_drm_private *priv = dev->dev_private;

    if (!priv->dma_coherent && etnaviv_obj->flags & ETNA_BO_CACHED) {
        /* fini without a prep is almost certainly a userspace error */
        WARN_ON(etnaviv_obj->last_cpu_prep_op == 0);
        dma_sync_sgtable_for_device(dev->dev, etnaviv_obj->sgt,
etnaviv_op_to_dma_dir(etnaviv_obj->last_cpu_prep_op));
        etnaviv_obj->last_cpu_prep_op = 0;
    }

    return 0;
}

```


But, this is acceptable, because we can kill the GEM_CPU_PREP and 
GEM_CPU_FINI ioctl entirely

at userspace for cached buffer, as this is totally not needed for cached 
mapping on our platform.


Well leave this for the WC mapping only,

OK ?

--
Jingfeng