Re: [PATCH] usb: dwc3: gadget: Update chain bit correctly when using sg list

[Jack Pham <jackp@codeaurora.org>]

Hi John,

Thanks for following-up with this! While you're doing minor tweaks
anyway, I hope you don't mind me picking some nits below.

On Tue, Feb 18, 2020 at 11:51:04PM +0000, John Stultz wrote:
> From: Pratham Pratap <prathampratap@codeaurora.org>
> 
> If scatter-gather operation is allowed, a large USB request is split
> into multiple TRBs. For preparing TRBs for sg list, driver iterates
> over the list and creates TRB for each sg and mark the chain bit to
> false for the last sg. The current IOMMU driver is clubbing the list
> of sgs which shares a page boundary into one and giving it to USB driver.
> With this the number of sgs mapped it not equal to the the number of sgs
> passed. Because of this USB driver is not marking the chain bit to false
> since it couldn't iterate to the last sg. This patch addresses this issue
> by marking the chain bit to false if it is the last mapped sg.
> 
> At a practical level, this patch resolves USB transfer stalls
> seen with adb on dwc3 based db845c, pixel3 and other qcom
> hardware after functionfs gadget added scatter-gather support
> around v4.20.
> 
> Credit also to Anurag Kumar Vulisha <anurag.kumar.vulisha@xilinx.com>
> who implemented a very similar fix to this issue.
> 
> Cc: Felipe Balbi <balbi@kernel.org>
> Cc: Yang Fei <fei.yang@intel.com>
> Cc: Thinh Nguyen <thinhn@synopsys.com>
> Cc: Tejas Joglekar <tejas.joglekar@synopsys.com>
> Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
> Cc: Jack Pham <jackp@codeaurora.org>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Greg KH <gregkh@linuxfoundation.org>
> Cc: Linux USB List <linux-usb@vger.kernel.org>
> Cc: stable <stable@vger.kernel.org>
> Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>
> [jstultz: Slight tweak to remove sg_is_last() usage, reworked
>           commit message, minor comment tweak]
> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
>  drivers/usb/dwc3/gadget.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 1b8014ab0b25..10aa511051e8 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -1071,7 +1071,14 @@ static void dwc3_prepare_one_trb_sg(struct dwc3_ep *dep,
>  		unsigned int rem = length % maxp;
>  		unsigned chain = true;
>  
> -		if (sg_is_last(s))
> +		/*
> +		 * IOMMU driver is coalescing the list of sgs which shares a
> +		 * page boundary into one and giving it to USB driver. With
> +		 * this the number of sgs mapped it not equal to the the number
                                                 ^^ s/it/is/     ^^^ /d

Or could we more specifically say "number of sgs mapped could be less
than number passed"?

> +		 * of sgs passed. Mark the chain bit to false if it is the last
> +		 * mapped sg.
> +		 */
> +		if ((i == remaining - 1))

These outer parens are superfluous.

Also wondering if it would be more or less clear to just set the
variable once (and awkwardly move the comment to appear above the
local var declaration):

		unsigned chain = (i < remaining - 1);

>  			chain = false;
>  
>  		if (rem && usb_endpoint_dir_out(dep->endpoint.desc) && !chain) {

Jack
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