linux-kernel - Re: [PATCH 2/2] mtd: rawnand: meson: add support for Amlogic NAND flash controller

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Date:   Thu, 19 Jul 2018 17:53:24 +0800
From:   Yixun Lan <yixun.lan@...ogic.com>
To:     Boris Brezillon <boris.brezillon@...tlin.com>
CC:     <yixun.lan@...ogic.com>, Rob Herring <robh@...nel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Richard Weinberger <richard@....at>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        <linux-kernel@...r.kernel.org>,
        Marek Vasut <marek.vasut@...il.com>,
        Jian Hu <jian.hu@...ogic.com>,
        Liang Yang <liang.yang@...ogic.com>,
        <linux-mtd@...ts.infradead.org>,
        Kevin Hilman <khilman@...libre.com>,
        Carlo Caione <carlo@...one.org>,
        <linux-amlogic@...ts.infradead.org>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Jerome Brunet <jbrunet@...libre.com>
Subject: Re: [PATCH 2/2] mtd: rawnand: meson: add support for Amlogic NAND
 flash controller


HI Boris


On 07/19/18 16:39, Boris Brezillon wrote:
> Hi Yixun,
> 
> On Thu, 19 Jul 2018 16:13:47 +0800
> Yixun Lan <yixun.lan@...ogic.com> wrote:
> 
>>>>> You're doing DMA on those buffers, and devm_kzalloc() is not
>>>>> DMA-friendly (returned buffers are not aligned on a cache line). Also,
>>>>> you don't have to allocate your own buffers because the core already
>>>>> allocate them (chip->data_buf, chip->oob_poi). All you need to do is
>>>>> set the NAND_USE_BOUNCE_BUFFER flag in chip->options to make sure
>>>>> you're always passed a DMA-able buffer.
>>>>>     
>>>>
>>>> thanks for the suggestion, we've migrated to use the
>>>> dmam_alloc_coherent() API  
>>>
>>> kzalloc() should be just fine, no need to alloc a DMA coherent region. 
>>>   
>>
>> we're a little bit confused here, isn't devm_kzalloc (previously we are
>> using) a variant of kzalloc? and since the NAND controller is doing DMA
>> here, using DMA coherent API is more proper way?
> 
> Well, making buffers DMA coherent might be expensive, especially if you
> access them a lot (unless you have a coherency unit and the cache is
> kept enabled).
> 
> Regarding the "why is devm_kzalloc() is not DMA-safe?" question, I'd
> recommend that you read this discussion [1].
> 
great, thanks for the info.

we fixed this in patch v2

>>>>>> +	mtd->name = devm_kasprintf(nfc->dev, GFP_KERNEL,
>>>>>> +				   "%s:nand", dev_name(dev));
>>>>>> +	if (!mtd->name) {
>>>>>> +		dev_err(nfc->dev, "Failed to allocate mtd->name\n");
>>>>>> +		return -ENOMEM;
>>>>>> +	}    
>>>>>
>>>>> You set the name after nand_scan_ident() and make it conditional (only
>>>>> if ->name == NULL) so that the label property defined in the DT takes
>>>>> precedence over the default name.    
>>>>  
>> for setting mtd->name conditional, do you mean doing something like this?
>>
>> if (!mtd->name)
>> 	mtd->name = devm_kasprintf(..)
> 
> Yes, that's what I meant.
> 
>>
>> but we found mtd->name = "ffe07800.nfc" after function
>> nand_scan_ident(), which is same value as dev_name(dev)..
>> and there is no cs information encoded there.
> 
> Hm, that shouldn't be the case. Maybe you can add traces to find out
> who is setting mtd->name to this value.
> 
will trace this, then get back to you
>>
>>>>  
>>>>> Also, I recommend suffixing this name
>>>>> with the CS id, just in case you ever need to support connecting several
>>>>> chips to the same controller. 
>>>>>     
>>>>
>>>> we actually didn't get the point here, cs is about chip selection with
>>>> multiple nand chip? and how to get this information?  
>>>
>>> Well, you currently seem to only support one chip per controller, but I
>>> guess the IP can handle several CS lines. So my recommendation is about
>>> choosing a name so that you can later easily add support for multiple
>>> chips without breaking setups where mtdparts is used.
>>>
>>> To sum-up, assuming your NAND chip is always connected to CS0 (on the
>>> controller side), I'd suggest doing:
>>>   
>> yes, this is exactly how the hardware connected.
>>> 	mtd->name = devm_kasprintf(nfc->dev, GFP_KERNEL,
>>> 				   "%s:nand.%d", dev_name(dev), cs_id);
>>>
>>> where cs_id is the value you extracted from the reg property of the
>>> NAND node.
>>>   
>> Ok, you right.
>> current, the NAND chip is only use one CS (which CE0) for now, what's in
>> the DT is
>>
>> nand@0 {
>>  reg = < 0 >;
>>  ..
>> };
>>
>> so for the multiple chips it would something like this in DT?
>>
>> nand@0 {
>>   reg = < 0 >;
>> };
>>
>> nand@1 {
>>   reg = < 1 >;
>> };
> 
> Yep, that's for 2 single-die chips.
> 
>>
>> or even
>> nand@0 {
>>   reg = < 0 2 >;
>> };
>>
>> nand@1 {
> 
> nand@3 {
> 
>>   reg = < 3 4 >;
>> };
> 
> And this is describing 2 dual-die chips.
> 
>>
>> do we need to encode all the cs information here? not sure if we
>> understand this correctly, but could send out the patch for review..
> 
> Yes, reg should contain an array of controller-side CS lines used to
> select the chip (or a specific die in a chip, the index in the reg
> table being the id of the die).
>
much clear about this, thanks

Yixun