linux-kernel - Re: [PATCH v2] usb: core: warn if a GFP zone flag is passed to hcd_buffer

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Message-ID: <65bd42ee-ecbc-4a23-9036-42358aa086df@suse.com>
Date: Wed, 16 Apr 2025 15:08:53 +0200
From: Oliver Neukum <oneukum@...e.com>
To: Petr Tesarik <ptesarik@...e.com>, Oliver Neukum <oneukum@...e.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] usb: core: warn if a GFP zone flag is passed to
 hcd_buffer_alloc()

On 16.04.25 12:47, Petr Tesarik wrote:
  
> If I stay with the USB buffer allocations, AFAICS the mem_flags
> parameter should be used only for non-zone flags. If you specify,
> GFP_DMA here, it will have no impact whatsoever on allocating DMA
> buffers. It may unnecessarily allocate from the DMA zone for doing PIO.

Yes. But we should not limit enforcement of such a _new_ policy
to one method in order to fix a hypothetical issue.
There is just no need for action.

> Now I think I should really write an article for LWN to debunk some
> myths about GFP_DMA.

Well, if you go to that trouble an explanation of why memflags
are passed in USB at all and how DMA works in general would be
productive.

	Regards
		Oliver