linux-kernel - Re: [PATCH v6 03/33] xhci: sideband: add initial api to register a sideband entity

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Date:   Tue, 19 Sep 2023 14:26:50 -0700
From:   Wesley Cheng <quic_wcheng@...cinc.com>
To:     Hillf Danton <hdanton@...a.com>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>
CC:     Greg KH <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        USB <linux-usb@...r.kernel.org>
Subject: Re: [PATCH v6 03/33] xhci: sideband: add initial api to register a
 sideband entity

Hi Hillf,

On 9/16/2023 2:02 AM, Hillf Danton wrote:
> On Fri, 15 Sep 2023 17:09:56 -0700 Wesley Cheng <quic_wcheng@...cinc.com>
>> +static int
>> +xhci_ring_to_sgtable(struct xhci_sideband *sb, struct xhci_ring *ring, struct device *dev)
>> +{
>> +	struct sg_table	*sgt;
>> +	struct xhci_segment *seg;
>> +	struct page **pages;
>> +	unsigned int n_pages;
>> +	size_t sz;
>> +	int i;
>> +
>> +	sz = ring->num_segs * TRB_SEGMENT_SIZE;
>> +	n_pages = PAGE_ALIGN(sz) >> PAGE_SHIFT;
>> +	pages = kvmalloc_array(n_pages, sizeof(struct page *), GFP_KERNEL);
>> +	if (!pages)
>> +		return 0;
>> +
>> +	sgt = kzalloc(sizeof(struct sg_table), GFP_KERNEL);
>> +	if (!sgt) {
>> +		kvfree(pages);
>> +		return 0;
>> +	}
>> +
>> +	seg = ring->first_seg;
>> +
>> +	/*
>> +	 * Rings can potentially have multiple segments, create an array that
>> +	 * carries page references to allocated segments.  Utilize the
>> +	 * sg_alloc_table_from_pages() to create the sg table, and to ensure
>> +	 * that page links are created.
>> +	 */
>> +	for (i = 0; i < ring->num_segs; i++) {
>> +		pages[i] = vmalloc_to_page(seg->trbs);
>> +		seg = seg->next;
>> +	}
> 
> Given dma_pool_zalloc() in xhci_segment_alloc() and dma_alloc_coherent() in
> pool_alloc_page(), it is incorrect to get page from the cpu address returned
> by the dma alloc routine.

Thanks for the review.  That's true...at least on my particular platform 
it was working, because it looks like the cpu addr returned from those 
dma calls was going through the path where we call iommu_dma_alloc() --> 
iommu_dma_alloc_remap().  However, not every implementation will have 
IOMMU involved either.

I'll change the logic to below, so that we can fetch the pages based on 
the IOVA/DMA address.

	for (i = 0; i < ring->num_segs; i++) {
		dma_get_sgtable(dev, sgt, seg->trbs, seg->dma,
					TRB_SEGMENT_SIZE);
		pages[i] = sg_page(sgt->sgl);
		sg_free_table(sgt);
		seg = seg->next;
	}

Thanks
Wesley Cheng