Re: [RFC PATCH v2 4/4] usb: xhci: Use temporary buffer to consolidate SG

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 21.4.2020 12.49, Tejas Joglekar wrote:
> The Synopsys xHC has an internal TRB cache of size TRB_CACHE_SIZE for
> each endpoint. The default value for TRB_CACHE_SIZE is 16 for SS and 8
> for HS. The controller loads and updates the TRB cache from the transfer
> ring in system memory whenever the driver issues a start transfer or
> update transfer command.
> 
> For chained TRBs, the Synopsys xHC requires that the total amount of
> bytes for all TRBs loaded in the TRB cache be greater than or equal to 1
> MPS. Or the chain ends within the TRB cache (with a last TRB).
> 
> If this requirement is not met, the controller will not be able to send
> or receive a packet and it will hang causing a driver timeout and error.
> 
> This can be a problem if a class driver queues SG requests with many
> small-buffer entries. The XHCI driver will create a chained TRB for each
> entry which may trigger this issue.
> 
> This patch adds logic to the XHCI driver to detect and prevent this from
> happening.
> 
> For every (TRB_CACHE_SIZE - 2), we check the total buffer size of
> the SG list and if the last window of (TRB_CACHE_SIZE - 2) SG list length
> and we don't make up at least 1 MPS, we create a temporary buffer to
> consolidate full SG list into the buffer.
> 
> We check at (TRB_CACHE_SIZE - 2) window because it is possible that there
> would be a link and/or event data TRB that take up to 2 of the cache
> entries.
> 
> We discovered this issue with devices on other platforms but have not
> yet come across any device that triggers this on Linux. But it could be
> a real problem now or in the future. All it takes is N number of small
> chained TRBs. And other instances of the Synopsys IP may have smaller
> values for the TRB_CACHE_SIZE which would exacerbate the problem.
> 
> Signed-off-by: Tejas Joglekar <joglekar@synopsys.com>
> ---
>  Changes in v2:
>  - Removed redundant debug messages
>  - Modified logic to remove unnecessary changes in hcd.c
>  - Rename the quirk
> 
>  drivers/usb/host/xhci-ring.c |   2 +-
>  drivers/usb/host/xhci.c      | 125 +++++++++++++++++++++++++++++++++++++++++++
>  drivers/usb/host/xhci.h      |   4 ++
>  3 files changed, 130 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> index a78787bb5133..2fad9474912a 100644
> --- a/drivers/usb/host/xhci-ring.c
> +++ b/drivers/usb/host/xhci-ring.c
> @@ -3291,7 +3291,7 @@ int xhci_queue_bulk_tx(struct xhci_hcd *xhci, gfp_t mem_flags,
>  
>  	full_len = urb->transfer_buffer_length;
>  	/* If we have scatter/gather list, we use it. */
> -	if (urb->num_sgs) {
> +	if (urb->num_sgs && !(urb->transfer_flags & URB_DMA_MAP_SINGLE)) {
>  		num_sgs = urb->num_mapped_sgs;
>  		sg = urb->sg;
>  		addr = (u64) sg_dma_address(sg);
> diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
> index fe38275363e0..15f06bc6b1ad 100644
> --- a/drivers/usb/host/xhci.c
> +++ b/drivers/usb/host/xhci.c
> @@ -1256,6 +1256,106 @@ EXPORT_SYMBOL_GPL(xhci_resume);
>  
>  /*-------------------------------------------------------------------------*/
>  
> +static int xhci_map_temp_buffer(struct usb_hcd *hcd, struct urb *urb)
> +{
> +	void *temp;
> +	int ret = 0;
> +	unsigned int len;
> +	unsigned int buf_len;
> +	enum dma_data_direction dir;
> +	struct xhci_hcd *xhci;
> +
> +	xhci = hcd_to_xhci(hcd);
> +	dir = usb_urb_dir_in(urb) ? DMA_FROM_DEVICE : DMA_TO_DEVICE;
> +	buf_len = urb->transfer_buffer_length;
> +
> +	temp = kzalloc_node(buf_len, GFP_ATOMIC,
> +			    dev_to_node(hcd->self.sysdev));
> +
> +	if (usb_urb_dir_out(urb))
> +		len = sg_pcopy_to_buffer(urb->sg, urb->num_sgs,
> +					 temp, buf_len, 0);
> +
> +	urb->transfer_buffer = temp;
> +	urb->transfer_dma = dma_map_single(hcd->self.sysdev,
> +					   urb->transfer_buffer,
> +					   urb->transfer_buffer_length,
> +					   dir);
> +
> +	if (dma_mapping_error(hcd->self.sysdev,
> +			      urb->transfer_dma)) {
> +		ret = -EAGAIN;
> +		kfree(temp);
> +	} else {
> +		urb->transfer_flags |= URB_DMA_MAP_SINGLE;

Not sure if using URB_DMA_MAP_SINGLE to flag that this urb is boucebuffered is
appropriate.

If Greg or Alan don't object then it's fine for me as well.



> +	}
> +
> +	return ret;
> +}
> +
> +static bool xhci_urb_temp_buffer_required(struct usb_hcd *hcd,
> +					  struct urb *urb)
> +{
> +	bool ret = false;
> +	unsigned int i;
> +	unsigned int len = 0;
> +	unsigned int buf_len;
> +	unsigned int trb_size;
> +	unsigned int max_pkt;
> +	struct scatterlist *sg;
> +	struct scatterlist *tail_sg;
> +
> +	sg = urb->sg;
> +	tail_sg = urb->sg;
> +	buf_len = urb->transfer_buffer_length;
> +	max_pkt = usb_endpoint_maxp(&urb->ep->desc);
> +
> +	if (!urb->num_sgs)
> +		return ret;
> +
> +	if (urb->dev->speed >= USB_SPEED_SUPER)
> +		trb_size = TRB_CACHE_SIZE_SS;
> +	else
> +		trb_size = TRB_CACHE_SIZE_HS;
> +
> +	if (urb->transfer_buffer_length != 0 &&
> +	    !(urb->transfer_flags & URB_NO_TRANSFER_DMA_MAP)) {
> +		for_each_sg(urb->sg, sg, urb->num_sgs, i) {
> +			len = len + sg->length;
> +			if (i > trb_size - 2) {
> +				len = len - tail_sg->length;
> +				if (len < max_pkt) {
> +					ret = true;
> +					break;
> +				}
> +
> +				tail_sg = sg_next(tail_sg);
> +			}
> +		}
> +	}
> +	return ret;
> +}
> +
> +static void xhci_unmap_temp_buf(struct urb *urb)
> +{
> +	struct scatterlist *sg;
> +	unsigned int len;
> +	unsigned int buf_len;
> +
> +	sg = urb->sg;
> +	buf_len = urb->transfer_buffer_length;
> +
> +	if (usb_urb_dir_in(urb)) {
> +		len = sg_pcopy_from_buffer(urb->sg, urb->num_sgs,
> +					   urb->transfer_buffer,
> +					   buf_len,
> +					   0);
> +	}
> +
> +	kfree(urb->transfer_buffer);
> +	urb->transfer_buffer = NULL;

clear URB_DMA_MAP_SINGLE from urb->transfer_flags?

Everything else looks good do me.
-Mathias