Re: read() via USB bus

On 10.08.21 16:13, Muni Sekhar wrote:

On Mon, Aug 9, 2021 at 1:45 PM Oliver Neukum [off-list ref] wrote:

quoted

On 09.08.21 09:58, Muni Sekhar wrote:

quoted

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.

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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