linux-kernel - Re: [PATCH] usbip: set the dma mask to 64bit default for vhci-driver

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Message-ID: <097ad0fd-db38-4174-8e34-4ceb485e7e23@linuxfoundation.org>
Date: Mon, 28 Apr 2025 17:07:23 -0600
From: Shuah Khan <skhan@...uxfoundation.org>
To: Greg KH <gregkh@...uxfoundation.org>, Zongmin Zhou <min_halo@....com>
Cc: Christoph Hellwig <hch@...radead.org>, i@...ithal.me,
 linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org, shuah@...nel.org,
 valentina.manea.m@...il.com, Zongmin Zhou <zhouzongmin@...inos.cn>,
 Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] usbip: set the dma mask to 64bit default for vhci-driver

On 4/28/25 04:04, Greg KH wrote:
> A: http://en.wikipedia.org/wiki/Top_post
> Q: Were do I find info about this thing called top-posting?
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
> 
> A: No.
> Q: Should I include quotations after my reply?
> 
> http://daringfireball.net/2007/07/on_top
> 
> On Mon, Apr 28, 2025 at 05:51:08PM +0800, Zongmin Zhou wrote:
>> Dear Greg and Shuah,
>>
>> I found out that the vhci-hcd driver added this virtual device
>> as a platform device from the very beginning since 2014.
> 
> Ah, I should have caught it back then, but at the time there really
> wasn't another option.
> 

>> I'm just getting in touch with this module and
>> don't have a deep understanding of it，shuah should be clearer.

faux_device should work fine for this. We do have to test of course.

There are several examples of converting  platform device to faux device.

72239a78f9f5b9f05ea4bb7a15b92807906dab71
dcd2a9a5550ef556c8fc11601a0f729fb71ead5d

> 
> See the recent patches I did converting drivers to use the faux bus
> code, it should be pretty simple to do.
> 
>> I don't know if using the faux bus to replace the platform bus can solve the
>> problem that the error limitation on max_hw_sectors for usbip device
>> since commit d74ffae8b8dd applied.
> 
> That is for the storage driver, not usbip.  As the faux bus does not
> have any real dma operations, this should cause it to work properly
> given the default values involed, but that's up to you to test to verify
> it does just that.  Try it and see!
> 
>> But this change will request user to update kernel version to support faux
>> bus.
> 
> That's just a normal kernel update to a newer version, what is wrong
> with that?

With one difference that the fix depends on faux_device feature - hence
we can't apply it to stables. I do think it is the right direction to
go to faux_device.

> 
>> This will also be an expensive change to fix the problem?
> 
> Fixing things properly is the correct thing to do in all cases.
> 

Zongmin, do let me know if you are unable to make the change.

thanks,
-- Shuah