From: Kirill A. Korinsky <kirill@korins.ky>
Subject: Re: Fix dumping ISOCHRONOUS IN transfers
To: tech@openbsd.org
Date: Fri, 20 Dec 2024 20:04:10 +0100

On Fri, 20 Dec 2024 19:40:02 +0100,
Martin Pieuchot <mpi@grenadille.net> wrote:
> 
> On 19/12/24(Thu) 14:18, Kirill A. Korinsky wrote:
> > [...] 
> > However, the situation is different for IN transfers. The kernel reads
> > packets from the device, but they may be shorter than the expected size. As
> > a result, the kernel tracks the total length of packets read using
> > xfer->actlen and updates xfer->frlengths[i] accordingly.
> 
> Understood.  I'd prefer if we could find another solution than adding an
> array to every USB transfer descriptor.
> It seems to me that all isochronous USB drivers use frames of the same size,
> which mean we could calculate the offset with:
> 
> 	(xfer->length / xfer->nframes)
> 
> Could that work?
> 
> On a related note, is their any OS and/or driver that use isochronous
> transfers with frames of different sizes?
> 

I made a quick grep through the Linux sources and was able to locate an
example with a different offset and length in a driver:
https://github.com/torvalds/linux/blob/v6.12/drivers/isdn/hardware/mISDN/hfcsusb.c#L1245-L1321

Linux uses the following structure to describe each isochronous packet:

        struct usb_iso_packet_descriptor {
                unsigned int offset;
                unsigned int length;		/* expected length */
                unsigned int actual_length;
                int status;
        };

Since we have a few isochronous drivers, what do you think about migrating
the code to follow the same approach? I mean that the driver should allocate
an array of usb_iso_packet_descriptor instead of an array of uint16_t.

-- 
wbr, Kirill