Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops

[Lu Baolu]

Hi,

On 11/14/19 4:14 PM, Christoph Hellwig wrote:
> On Thu, Nov 14, 2019 at 01:14:11PM +0800, Lu Baolu wrote:
> > Could you please educate me what dma_supported() is exactly for? Will
> > it always get called during boot? When will it be called?
>
> ->dma_supported is set when setting either the dma_mask or
> dma_coherent_mask. These days it serves too primary purposes: reject
> too small masks that can't be addressed, and provide any hooks needed
> in the driver based on the mask.

Thanks! So ->dma_supported might not be called before driver maps buffer
and start DMA. Right?

> > In above implementation, why do we need to check dma_direct_supported()
> > at the beginning? And why
>
> Because the existing driver called dma_direct_supported, which I added
> based on x86 arch overrides doings the same a while ago.  I suspect
> it is related to addressing for tiny dma masks, but I'm not entirely
> sure.  The longer term intel-iommu maintainers or x86 maintainers might
> be able to shed more light how this was supposed to work and/or how
> systems with the Intel IOMMU deal with e.g. ISA devices with 24-bit
> addressing.

Yes. Make sense.

> > 	if (!info || info == DUMMY_DEVICE_DOMAIN_INFO ||
> > 			info == DEFER_DEVICE_DOMAIN_INFO) {
> > 		dev->dma_ops_bypass = true;
>
> This was supposed to transform the checks from iommu_dummy and
> identity_mapping.  But I think it actually isn't entirely correct and
> already went bad in the patch to remove identity_mapping.  Pleae check
> the branch I just re-pushed, which should be correct now.

Okay. Thanks!

Best regard,
baolu