Re: read() via USB bus

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

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?

USB devices are primarily based on endpoints, not registers. A literal
answer to your question would point you to the clear/set/get_feature
standard requests of chapter 9 of the specification, but that really
will not help you, as you are making assumption that fundamentally
do not apply.

I hope this list stays friendly to newcomers and we will answer
specific questions, but at this point I must advise you to first
read an introductory book.

    HTH
        Oliver

-- 
Thanks,
Sekhar

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