linux-kernel - Re: [PATCH 2/2] usb: cdns3: Optimize DMA request buffer allocation

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Message-ID: <20210309103116.GA110148@infradead.org>
Date:   Tue, 9 Mar 2021 10:31:16 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Sanket Parmar <sparmar@...ence.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "peter.chen@...nel.org" <peter.chen@...nel.org>,
        Pawel Laszczak <pawell@...ence.com>,
        "a-govindraju@...com" <a-govindraju@...com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rahul Kumar <kurahul@...ence.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "kishon@...com" <kishon@...com>
Subject: Re: [PATCH 2/2] usb: cdns3: Optimize DMA request buffer allocation

On Tue, Mar 09, 2021 at 10:18:43AM +0000, Sanket Parmar wrote:
> > On Tue, Mar 09, 2021 at 06:19:40AM +0100, Sanket Parmar wrote:
> > > dma_alloc_coherent() might fail on the platform with a small DMA region.
> > >
> > > To avoid such failure in cdns3_prepare_aligned_request_buf(),
> > > dma_alloc_coherent() is replaced with kmalloc and dma_map API to
> > > allocate aligned request buffer of dynamic length.
> > 
> > dma_alloc_noncoherent is the proper API instead of using kmalloc, which
> > can lead to unaddressable memory that might require bounce buffering.
> 
> cdns3 device required DMA coherent buffer to perform operations. So 
> dma_alloc_noncoherent will not help here.
> 
> Also all gadget classes(except g_ether) use kmalloc to allocated request buffer,
> and device driver uses usb_gadget_map_request_by_dev to map the request
> buffer. Similar approach is used to allocate aligned buffer. 

If you can use kmalloc and dma_map_single you can use
dma_alloc_noncoherent per definition.