Re: [PATCH v3] usb: gadget: f_midi: Add checking if it need align buffer's size to an ep's maxpacketsize

[Felipe Ferreri Tonello]

Hi Baolin,

Sorry for not replying for previous emails because for some reason these
emails were archived. :(

On 12/07/16 10:01, Baolin Wang wrote:
> Some gadget device (such as dwc3 gadget) requires quirk_ep_out_aligned_size
> attribute, which means it need to align the request buffer's size to an ep's
> maxpacketsize.
> 
> Thus we add usb_ep_align_maybe() function to check if it is need to align
> the request buffer's size to an ep's maxpacketsize.
> 
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxx>
> Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
> ---
> Changelog:
> v2:
>  - Simplify the method to get buffer length.
> v1:
>  - Remove the in_ep modification.
>  - Remove max_t() function.
> 
>  drivers/usb/gadget/function/f_midi.c |   11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_midi.c b/drivers/usb/gadget/function/f_midi.c
> index 58fc199..3486941 100644
> --- a/drivers/usb/gadget/function/f_midi.c
> +++ b/drivers/usb/gadget/function/f_midi.c
> @@ -359,10 +359,13 @@ static int f_midi_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
>  
>  	/* allocate a bunch of read buffers and queue them all at once. */
>  	for (i = 0; i < midi->qlen && err == 0; i++) {
> -		struct usb_request *req =
> -			midi_alloc_ep_req(midi->out_ep,
> -				max_t(unsigned, midi->buflen,
> -					bulk_out_desc.wMaxPacketSize));
> +		struct usb_request *req;
> +		unsigned length;
> +
> +		length = usb_ep_align_maybe(midi->gadget, midi->out_ep,
> +					    midi->buflen);
> +
> +		req = midi_alloc_ep_req(midi->out_ep, length);
>  		if (req == NULL)
>  			return -ENOMEM;
>  
> 

Yes, as mentioned before, my intent was to align the size otherwise an
actual nasty bug happens.

I have two problems with this approach:
* usb_ep_align_maybe() has a bug that it doesn't convert wMaxPacketSize
endianness to the CPU. See my patch on that[1] subject. Also this
function uses round_up which expects x and y to be a power of 2, is that
a feasible assumption?
* If the gadget driver doesn't support quirk_ep_out_aligned_size,
usb_ep_align_maybe() can potentially return a len < wMaxPacketSize.
Basically causing a regression.

I believe we should protect this bad behavior on alloc_ep_req() in
drivers/usb/gadget/u_f.c by forcing align the size if the endpoint in
subject is OUT.

[1] [PATCH 1/4] usb: gadget: f_midi: fixed endianness when using
wMaxPacketSize: https://lkml.org/lkml/2016/7/25/1028

-- 
Felipe

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