On 1/26/23 14:59, Troels Liebe Bentsen wrote:
On Thu, 26 Jan 2023 at 14:12, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
I would like to change /sys/bus/usb/devices/usbX/descriptors so it never blocks.
Patches gladly reviewed to do so :)
Be careful. Been there done that for FreeBSD. You can cache the
descriptor in memory - yes, but beware that the values inside the device
descriptor may change after re-enumerating the device via software, like
firmware upgrade, and that directly hits on the XHCI controller
programming, that you need to load and configure the new bMaxPacketSize
in there!
And the same goes for the other fields in there :-)
We will have a look and get back to you.
It's probably best to find the undocumented bits of your USB peripheral
controller first! With USB control transactions I've seen so much
craziness over the years you won't believe it. The only ones that get it
right, is the ones that lay out all USB control endpoint jobs in memory
via DMA descriptors. All the ones that simply use a few registers to
receive the SETUP packet, DATA and status ZLP, have undocumented races.
By races I mean, what happens if you get SETUP and DATA interrupt bits
at the same time, or maybe all three, what is the right order, or what
about STALL conditions and short control transfers and blah blah blah.
This thing can really blow your mind, but yeah, many device side
programmers simply use the example code they get from the vendor and
give a shit about anything that can later go wrong. That is my simple
impression so far in the USB world.
--HPS
thanks,
greg k-h
Regards Troels
