linux-kernel - Re: [PATCH v5 2/6] mtd: m25p80: add support of SPI 1-2-2 and 1-4-4 protocols

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Message-ID: <10463831-9caf-f9df-ea15-2b77e01bd4b5@gmail.com>
Date:   Sun, 9 Apr 2017 22:46:30 +0200
From:   Marek Vasut <marek.vasut@...il.com>
To:     Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        linux-mtd@...ts.infradead.org, jartur@...ence.com,
        kdasu.kdev@...il.com, mar.krzeminski@...il.com
Cc:     boris.brezillon@...e-electrons.com, richard@....at,
        nicolas.ferre@...rochip.com, linux-kernel@...r.kernel.org,
        computersforpeace@...il.com, dwmw2@...radead.org
Subject: Re: [PATCH v5 2/6] mtd: m25p80: add support of SPI 1-2-2 and 1-4-4
 protocols

On 04/09/2017 09:37 PM, Cyrille Pitchen wrote:
> Hi Marek,
> 
> Le 07/04/2017 à 01:37, Marek Vasut a écrit :
>> On 03/23/2017 12:33 AM, Cyrille Pitchen wrote:
>>> Before this patch, m25p80_read() supported few SPI protocols:
>>> - regular SPI 1-1-1
>>> - SPI Dual Output 1-1-2
>>> - SPI Quad Output 1-1-4
>>> On the other hand, m25p80_write() only supported SPI 1-1-1.
>>>
>>> This patch updates both m25p80_read() and m25p80_write() functions to let
>>> them support SPI 1-2-2 and SPI 1-4-4 protocols for Fast Read and Page
>>> Program SPI commands.
>>>
>>> It adopts a conservative approach to avoid regressions. Hence the new
>>                                              ^ FYI, regression != bug
>>
>>> implementations try to be as close as possible to the old implementations,
>>> so the main differences are:
>>> - the tx_nbits values now being set properly for the spi_transfer
>>>   structures carrying the (op code + address/dummy) bytes
>>> - and the spi_transfer structure being split into 2 spi_transfer
>>>   structures when the numbers of I/O lines are different for op code and
>>>   for address/dummy byte transfers on the SPI bus.
>>>
>>> Besides, the current spi-nor framework supports neither the SPI 2-2-2 nor
>>> the SPI 4-4-4 protocols. So, for now, we don't need to update the
>>> m25p80_{read|write}_reg() functions as SPI 1-1-1 is the only one possible
>>> protocol.
>>>
>>> Signed-off-by: Cyrille Pitchen <cyrille.pitchen@...el.com>
>>> ---
>>>  drivers/mtd/devices/m25p80.c | 120 ++++++++++++++++++++++++++++++++-----------
>>>  1 file changed, 90 insertions(+), 30 deletions(-)
>>>
>>> diff --git a/drivers/mtd/devices/m25p80.c b/drivers/mtd/devices/m25p80.c
>>> index 68986a26c8fe..64d562efc25d 100644
>>> --- a/drivers/mtd/devices/m25p80.c
>>> +++ b/drivers/mtd/devices/m25p80.c
>>> @@ -34,6 +34,19 @@ struct m25p {
>>>  	u8			command[MAX_CMD_SIZE];
>>>  };
>>>  
>>> +static inline void m25p80_proto2nbits(enum spi_nor_protocol proto,
>>> +				      unsigned int *inst_nbits,
>>> +				      unsigned int *addr_nbits,
>>> +				      unsigned int *data_nbits)
>>> +{
>>
>> Why don't we just have some generic macros to extract the number of bits
>> from $proto ?
>>
> 
> from Documentation/process/coding-style.rst:
> "Generally, inline functions are preferable to macros resembling functions."
> 
> inline functions provide better type checking of their arguments and/or
> returned value than macros.
> 
> Type checking is also the reason I have chosen to create the 'enum
> spi_nor_protocol' rather than using constant macros.

That part I get (no, not really [1], inline is compiler _hint_ and for
static function, the compiler is smart enough to figure out it should
inline it, so drop it. Also cf. __always_inline).

What I don't quite get is why don't we just encode the proto as ie.

#define PROTO_1_1_4 0x00010204 /* (== BIT(16) | BIT(8) | BIT(2)) */

in which case this whole function would turn into constant-time

return (proto >> (n * 8)) & 0xff;

where n is 0 for data, 1 for address , 2 for command .

[1] https://lwn.net/Articles/166172/

>>> +	if (inst_nbits)
>>> +		*inst_nbits = spi_nor_get_protocol_inst_width(proto);
>>> +	if (addr_nbits)
>>> +		*addr_nbits = spi_nor_get_protocol_addr_width(proto);
>>> +	if (data_nbits)
>>> +		*data_nbits = spi_nor_get_protocol_data_width(proto);
>>> +}
>>> +
[...]

-- 
Best regards,
Marek Vasut