read() via USB bus

[Muni Sekhar]

On Tue, 10 Aug, 2021, 7:50 pm Oliver Neukum, <oneukum at suse.com> wrote:

>
> On 10.08.21 16:13, Muni Sekhar wrote:
> > On Mon, Aug 9, 2021 at 1:45 PM Oliver Neukum <oneukum at suse.com> wrote:
> >>
> >> On 09.08.21 09:58, Muni Sekhar wrote:
> >>> Hi all,
> >>>
> >>> PCIe memory mapped registers can be read via readb(), readw(), readl()
> >>> kernel API's. Similarly what are the kernel API to read the device
> >>> registers via USB bus
> >>>
> >> Hi,
> >>
> >> I am afraid this is based on a fundamental misunderstanding on how
> >> USB works. It is based on passing messages, not reading and writing
> >> registers.
> > I am referring to the code mentioned in
> >
> https://patchwork.kernel.org/project/linux-arm-msm/patch/1534464348-8227-3-git-send-email-pheragu@codeaurora.org/
> >
> > As per this driver gets access to the Qcomm’s USB h/w device registers
> > via devm_extcon_dev_allocate(), devm_extcon_dev_register(),
> > platform_get_resource() and devm_ioremap_resource API’s.
> >
> > What does the USB external connector EXTCON_USB \ EXTCON_USB_HOST
> > devices means? Are these different from normal USB devices?
> >
> Hi,
>
> those are not USB devices. Those are devices associated with a USB bus
> and are to be
> found on the host's CPU's bus. This is like a graphics card is from the
> driver's view
> not a DisplayPort device or a SCSI controller is not a SCSI device for
> its driver.

Any other examples of devices associated with the USB bus? Do we have any
reference driver in the mainline kernel tree?
Does lsusb reports these devices?

>
> A host controller and associated devices can be on any bus. Such
> controllers follow
> their separate specifications and how they are to be driven is strictly
> speaking
> not part of USB:
>
>     Regards
>         Oliver
>
>
>
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