Re: [RFC/PATCH 14/45] media: usb: uvc: make use of new usb_endpoint_maxp_mult()

[Felipe Balbi <felipe.balbi@linux.intel.com>]

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Hi,

Laurent Pinchart <laurent.pinchart@ideasonboard.com> writes:
> Hi Felipe,
>
> Thanks for the patch.
>
> On Wednesday 28 Sep 2016 16:05:23 Felipe Balbi wrote:
>> We have introduced a helper to calculate multiplier
>> value from wMaxPacketSize. Start using it.
>> 
>> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
>> Cc: <linux-media@vger.kernel.org>
>> Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
>> ---
>>  drivers/media/usb/uvc/uvc_video.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/media/usb/uvc/uvc_video.c
>> b/drivers/media/usb/uvc/uvc_video.c index b5589d5f5da4..11e0e5f4e1c2 100644
>> --- a/drivers/media/usb/uvc/uvc_video.c
>> +++ b/drivers/media/usb/uvc/uvc_video.c
>> @@ -1467,6 +1467,7 @@ static unsigned int uvc_endpoint_max_bpi(struct
>> usb_device *dev, struct usb_host_endpoint *ep)
>>  {
>>  	u16 psize;
>> +	u16 mult;
>> 
>>  	switch (dev->speed) {
>>  	case USB_SPEED_SUPER:
>> @@ -1474,7 +1475,8 @@ static unsigned int uvc_endpoint_max_bpi(struct
>> usb_device *dev, return le16_to_cpu(ep->ss_ep_comp.wBytesPerInterval);
>>  	case USB_SPEED_HIGH:
>>  		psize = usb_endpoint_maxp(&ep->desc);
>> -		return (psize & 0x07ff) * (1 + ((psize >> 11) & 3));
>> +		mult = usb_endpoint_maxp_mult(&ep->desc);
>> +		return (psize & 0x07ff) * mult;
>
> I believe you can remove the & 0x07ff here after patch 28/45.
>
> This being said, wouldn't it be useful to introduce a helper function that 
> performs the full computation instead of requiring usage of two helpers that 
> both call __le16_to_cpu() on the same field ? Or possibly turn the whole 
> uvc_endpoint_max_bpi() function into a core helper ? I haven't checked whether 
> it can be useful to other drivers though.

it's probably not useful for many other drivers. The multiplier is only
valid for High-speed Isochronous and Interrupt endpoints. If it's not
High speed, we don't care. If it's not Isoc or Int, we don't care.

If we have a single helper, we are likely gonna be doing extra
computation for no reason. Moreover, this is only called once during
probe(), there's really nothing to optimize here.

-- 
balbi

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