linux-kernel - Re: [PATCH v2] char: xillybus: Check endpoint type before allocing

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Message-ID: <628CCE44.6020007@gmail.com>
Date:   Tue, 24 May 2022 15:23:32 +0300
From:   Eli Billauer <eli.billauer@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     Zheyu Ma <zheyuma97@...il.com>, arnd@...db.de,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] char: xillybus: Check endpoint type before allocing

On 23/05/22 20:15, Greg KH wrote:
>> Rather, it works like this: The driver uses the EP 1 IN and OUT endpoints to
>> >  query the device for a data structure, which contains information on the
>> >  device's communication channels. The driver sets up device files
>> >  accordingly, and it also gets informed on which bulk OUT endpoints are
>> >  present.
>> >  
>> >  For what it's worth, I think it's fairly safe to assume that if a device
>> >  returns a legal data structure (which passes a CRC test), it's a XillyUSB
>> >  device.
>>      
> Why?  You still need to verify that the requested endpoints match up
> with what the device really has.
>    
OK. So to summarize:

EP 1 IN and EP 1 OUT are always present in XillyUSB devices.

On top of these, there might be additional bulk OUT endpoints. The 
driver resolves which ones in setup_channels(), by scanning a data blob 
it has received from the device. This takes place in the for-loop inside 
this function. When (out_desc & 0x80) is true for a given @i in the 
loop, the device has a bulk OUT endpoint with address i+2 (e.g. if this 
condition is met for i==2, the device has a bulk OUT EP 4).

So it seems like setup_channels() would be the right place to make this 
check, since it's called during the device's probe.

I guess it would likewise make sense to check for EP 1 IN and OUT in 
xillyusb_setup_base_eps().

Thanks,
    Eli