linux-kernel - Re: [PATCH v5 2/3] dmaengine: mediatek: Add MediaTek High-Speed DMA controller for MT7622 and MT7623 SoC

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Message-ID: <1519984283.8089.182.camel@mtkswgap22>
Date:   Fri, 2 Mar 2018 17:51:23 +0800
From:   Sean Wang <sean.wang@...iatek.com>
To:     Vinod Koul <vinod.koul@...el.com>
CC:     <dan.j.williams@...el.com>, <robh+dt@...nel.org>,
        <mark.rutland@....com>, <dmaengine@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-mediatek@...ts.infradead.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Julia Lawall <julia.lawall@...6.fr>
Subject: Re: [PATCH v5 2/3] dmaengine: mediatek: Add MediaTek High-Speed DMA
 controller for MT7622 and MT7623 SoC

On Fri, 2018-03-02 at 13:47 +0530, Vinod Koul wrote:
> On Fri, Mar 02, 2018 at 02:47:51PM +0800, Sean Wang wrote:
> > Hi, Vinod
> > 
> > On Thu, 2018-03-01 at 18:26 +0530, Vinod Koul wrote:
> > > On Thu, Mar 01, 2018 at 06:27:01PM +0800, Sean Wang wrote:
> > > > On Thu, 2018-03-01 at 13:53 +0530, Vinod Koul wrote:
> > > > > On Sun, Feb 18, 2018 at 03:08:30AM +0800, sean.wang@...iatek.com wrote:
> > > > > 
> > > > > > @@ -0,0 +1,1054 @@
> > > > > > +// SPDX-License-Identifier: GPL-2.0
> > > > > // Copyright ...
> > > > > 
> > > > > The copyright line needs to follow SPDX tag line
> > > > > 
> > > > 
> > > > okay, I will make it reorder and be something like that
> > > > 
> > > > // SPDX-License-Identifier: GPL-2.0
> > > > /*
> > > >  * Copyright (c) 2017-2018 MediaTek Inc.
> > > >  * Author: Sean Wang <sean.wang@...iatek.com>
> > > >  *
> > > >  * Driver for MediaTek High-Speed DMA Controller
> > > >  *
> > > >  */
> > > 
> > > It needs to be:
> > > 
> > > // SPDX-License-Identifier: GPL-2.0
> > > // Copyright (c) 2017-2018 MediaTek Inc.
> > > 
> > > /*
> > >  * whatever else you want
> > >  */
> > > 
> > > The first two lines are in C99 style comment and need to have SPDX tag and
> > > Copyright info
> > 
> > Sure, I can do it using C99 style comments at the first two lines.
> > 
> > In addition, I'm really curious where we can find a reference to the
> > rule and if it 's a strict rule for all the drivers.
> > 
> > Because I'm considering whether I should turn other driver into using
> > the same rule.
> 
> Yes that seems to be the rule now https://lkml.org/lkml/2017/11/2/715
> 

Where could I find the rule for the copyright line needed to follow SPDX
tag line ?

currently, I still seen tons of drivers putting its copyright inside 
/* ... 
 * 
 */

> > > > > > +#define MTK_HSDMA_USEC_POLL		20
> > > > > > +#define MTK_HSDMA_TIMEOUT_POLL		200000
> > > > > > +#define MTK_HSDMA_DMA_BUSWIDTHS		BIT(DMA_SLAVE_BUSWIDTH_UNDEFINED)
> > > > > 
> > > > > Undefined buswidth??
> > > 
> > > ??
> > 
> > Sorry for I didn't answer the question in the short time.
> > 
> > After spending some time on a confirmation with design, it is
> > DMA_SLAVE_BUSWIDTH_4_BYTES and not be configurable. 
> 
> Then it should be DMA_SLAVE_BUSWIDTH_4_BYTES and not
> DMA_SLAVE_BUSWIDTH_UNDEFINED...
> 

understood, I will do it.

> > > > > shouldn't we check if next is in range, we can crash if we get bad value
> > > > > from hardware..
> > > > 
> > > > okay, there are checks for next with ddone bit check and null check in
> > > > the corresponding descriptor as the following.
> > > 
> > > what if you get bad next value
> > > 
> > 
> > next is not hardware value. it's maintained by software which is always
> > between 0 to MTK_DMA_SIZE - 1, and definitely doesn't get a bad value.
> > 
> > > > 
> > > > > > +		rxd = &pc->ring.rxd[next];
> > > 
> > > resulting in bad ref here
> > 
> > rxd is also definitely a good ref
> 
> not if next is out of range, say you read -1 or 200000?
> 

next must be in range because next is calculated by
MTK_HSDMA_NEXT_DESP_IDX macro which is just a modulo operation
with MTK_DMA_SIZE.

Currently, MTK_DMA_SIZE is equal to 64 and thus next must be 0 to 63 and
wraparound.

As to MTK_HSDMA_NEXT_DESP_IDX , it is defined as the following
#define MTK_HSDMA_NEXT_DESP_IDX(x, y)   (((x) + 1) & ((y) - 1))