netdev - Re: [net-next 15/19] can: tcan4x5x: rework SPI access

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Date:   Tue, 12 Jan 2021 16:36:31 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     David Laight <David.Laight@...LAB.COM>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        Dan Murphy <dmurphy@...com>, Sean Nyekjaer <sean@...nix.com>
Subject: Re: [net-next 15/19] can: tcan4x5x: rework SPI access

On 1/8/21 10:53 PM, David Laight wrote:
>> On 1/7/21 8:06 PM, Jakub Kicinski wrote:
>>> On Thu, 7 Jan 2021 11:00:35 -0800 Jakub Kicinski wrote:
>>>> On Thu,  7 Jan 2021 10:48:56 +0100 Marc Kleine-Budde wrote:
>>>>> +struct __packed tcan4x5x_map_buf {
>>>>> +	struct tcan4x5x_buf_cmd cmd;
>>>>> +	u8 data[256 * sizeof(u32)];
>>>>> +} ____cacheline_aligned;
>>>>
>>>> Interesting attribute combo, I must say.
>>>
>>> Looking at the rest of the patch I don't really see a reason for
>>> __packed.  Perhaps it can be dropped?
>>
>> It's the stream of bytes send via SPI to the chip. Here are both structs for
>> reference:
>>
>>> +struct __packed tcan4x5x_buf_cmd {
>>> +	u8 cmd;
>>> +	__be16 addr;
>>> +	u8 len;
>>> +};
>>
>> This has to be packed, as I assume the compiler would add some space after the
>> "u8 cmd" to align the __be16 naturally.
> 
> Why not generate a series of 32bit words to be sent over the SPI bus.
> Slightly less faffing in the code.
> Then have a #define (or inline function) to merge the cmd+addr+len
> into a single 32bit word.

The driver uses regmap. With proper configuration regmap already formats the
"cmd" and the "addr" in correct byte order into the first 3 bytes of the "reg_buf".

As regmap doesn't support generating a length parameter, I use the above struct
tcan4x5x_buf_cmd to set the length.

> Also if the length is in 32bit units, then the data[] field
> ought to be u32[].

In the regmap callback the data is passed with a void pointer and a length (in
bytes), so copying this to a "u8 data[]" felt more natural to me.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |



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