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Re: [PATCH v1] usb: Add read support for HCIVERSION register to XHCI

From: Cameron Esfahani
Subject: Re: [PATCH v1] usb: Add read support for HCIVERSION register to XHCI
Date: Tue, 31 Mar 2020 03:24:28 -0700

Actually, reading the specification a little more, the only fields which are 
documented as being smaller than 4-bytes are CAPLENGTH (1-byte) and HCIVERSION 

So maybe change impl.min_access_size to 1 and rely on the fact that 
traditionally callers haven't read less than 4-bytes for any of the 4-byte 

Cameron Esfahani

"In the elder days of Art, Builders wrought with greatest care each minute and 
unseen part; For the gods see everywhere."

"The Builders", H. W. Longfellow

> On Mar 31, 2020, at 2:57 AM, Cameron Esfahani via <address@hidden> wrote:
> Philippe -
> From what I've seen, access size has nothing to do with alignment.
> What the code in access_with_adjusted_size() will do is make sure that "size" 
> is >= min_access_size and <= max_access_size.
> So reading 2-bytes from address 2 turns into reading 4-bytes from address 2: 
> xhci_cap_read() is called with reg 2, size 4.
> But, due to the fact our change to support reg 2 only returns back 2-bytes, 
> and how the loops work in access_with_adjusted_size(), we only call 
> xhci_cap_read() once.
> It seems like we should also change impl.min_access_size for xhci_cap_ops to 
> be 2.
> But, after that, to support people doing strange things like reading 
> traditionally 4-byte values as 2 2-byte values, we probably need to change 
> xhci_cap_read() to handle every memory range in steps of 2-bytes.
> But I'll defer to Gerd on this...

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