Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 9 Mar 2021 10:31:16 +0000
From:   Christoph Hellwig <hch@infradead.org>
To:     Sanket Parmar <sparmar@cadence.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        "peter.chen@kernel.org" <peter.chen@kernel.org>,
        Pawel Laszczak <pawell@cadence.com>,
        "a-govindraju@ti.com" <a-govindraju@ti.com>,
        "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Rahul Kumar <kurahul@cadence.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "kishon@ti.com" <kishon@ti.com>
Subject: Re: [PATCH 2/2] usb: cdns3: Optimize DMA request buffer allocation
Message-ID: <20210309103116.GA110148@infradead.org>
References: <1615267180-9289-1-git-send-email-sparmar@cadence.com>
 <1615267180-9289-2-git-send-email-sparmar@cadence.com>
 <20210309092804.GA76839@infradead.org>
 <BY5PR07MB81197BAC5B801816D2B2566CB0929@BY5PR07MB8119.namprd07.prod.outlook.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <BY5PR07MB81197BAC5B801816D2B2566CB0929@BY5PR07MB8119.namprd07.prod.outlook.com>
Precedence: bulk

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.