Message-ID: <949d1f87-fcf3-55a2-56d7-e700379d4c00@wedev4u.fr>
Date:   Thu, 20 Apr 2017 01:17:03 +0200
From:   Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>
To:     Marek Vasut <marek.vasut@...il.com>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        linux-mtd@...ts.infradead.org
Cc:     boris.brezillon@...e-electrons.com, computersforpeace@...il.com,
        dwmw2@...radead.org, linux-kernel@...r.kernel.org, richard@....at
Subject: Re: [PATCH v7 1/4] mtd: spi-nor: introduce SPI 1-2-2 and SPI 1-4-4
 protocols

Le 19/04/2017 à 23:31, Marek Vasut a écrit :
> On 04/19/2017 10:12 PM, Cyrille Pitchen wrote:
>> Le 19/04/2017 à 01:02, Marek Vasut a écrit :
>>> On 04/19/2017 12:51 AM, Cyrille Pitchen wrote:
>>>> This patch changes the prototype of spi_nor_scan(): its 3rd parameter
>>>> is replaced by a 'struct spi_nor_hwcaps' pointer, which tells the spi-nor
>>>> framework about the actual hardware capabilities supported by the SPI
>>>> controller and its driver.
>>>>
>>>> Besides, this patch also introduces a new 'struct spi_nor_flash_parameter'
>>>> telling the spi-nor framework about the hardware capabilities supported by
>>>> the SPI flash memory and the associated settings required to use those
>>>> hardware caps.
>>>>
>>>> Then, to improve the readability of spi_nor_scan(), the discovery of the
>>>> memory settings and the memory initialization are now split into two
>>>> dedicated functions.
>>>>
>>>> 1 - spi_nor_init_params()
>>>>
>>>> The spi_nor_init_params() function is responsible for initializing the
>>>> 'struct spi_nor_flash_parameter'. Currently this structure is filled with
>>>> legacy values but further patches will allow to override some parameter
>>>> values dynamically, for instance by reading the JESD216 Serial Flash
>>>> Discoverable Parameter (SFDP) tables from the SPI memory.
>>>> The spi_nor_init_params() function only deals with the hardware
>>>> capabilities of the SPI flash memory: especially it doesn't care about
>>>> the hardware capabilities supported by the SPI controller.
>>>>
>>>> 2 - spi_nor_setup()
>>>>
>>>> The second function is called once the 'struct spi_nor_flash_parameter'
>>>> has been initialized by spi_nor_init_params().
>>>> With both 'struct spi_nor_flash_parameter' and 'struct spi_nor_hwcaps',
>>>> the new argument of spi_nor_scan(), spi_nor_setup() computes the best
>>>> match between hardware caps supported by both the (Q)SPI memory and
>>>> controller hence selecting the relevant settings for (Fast) Read and Page
>>>> Program operations.
>>>>
>>>> Signed-off-by: Cyrille Pitchen <cyrille.pitchen@...el.com>
>>>
>>> [...]
>>>
>>>> --- a/include/linux/mtd/spi-nor.h
>>>> +++ b/include/linux/mtd/spi-nor.h
>>>> @@ -119,13 +119,57 @@
>>>>  /* Configuration Register bits. */
>>>>  #define CR_QUAD_EN_SPAN		BIT(1)	/* Spansion Quad I/O */
>>>>  
>>>> -enum read_mode {
>>>> -	SPI_NOR_NORMAL = 0,
>>>> -	SPI_NOR_FAST,
>>>> -	SPI_NOR_DUAL,
>>>> -	SPI_NOR_QUAD,
>>>> +/* Supported SPI protocols */
>>>> +#define SNOR_PROTO_INST_MASK	GENMASK(23, 16)
>>>> +#define SNOR_PROTO_INST_SHIFT	16
>>>> +#define SNOR_PROTO_INST(_nbits)	\
>>>> +	((((u32)(_nbits)) << SNOR_PROTO_INST_SHIFT) & SNOR_PROTO_INST_MASK)
>>>
>>> Is the u32 cast needed ?
>>>
>>>> +#define SNOR_PROTO_ADDR_MASK	GENMASK(15, 8)
>>>> +#define SNOR_PROTO_ADDR_SHIFT	8
>>>> +#define SNOR_PROTO_ADDR(_nbits)	\
>>>> +	((((u32)(_nbits)) << SNOR_PROTO_ADDR_SHIFT) & SNOR_PROTO_ADDR_MASK)
>>>> +
>>>> +#define SNOR_PROTO_DATA_MASK	GENMASK(7, 0)
>>>> +#define SNOR_PROTO_DATA_SHIFT	0
>>>> +#define SNOR_PROTO_DATA(_nbits)	\
>>>> +	((((u32)(_nbits)) << SNOR_PROTO_DATA_SHIFT) & SNOR_PROTO_DATA_MASK)
>>>
>>> [...]
>>>
>>>> +static inline u8 spi_nor_get_protocol_inst_nbits(enum spi_nor_protocol proto)
>>>> +{
>>>> +	return ((u32)(proto & SNOR_PROTO_INST_MASK)) >> SNOR_PROTO_INST_SHIFT;
>>>
>>> DTTO, is the cast needed ?
>>>
>>
>> # Short answer:
>>
>> not necessary in this very particular case but always a good practice.
>>
>>
>>
>> # Comprehensive comparison with macro definitions:
>>
>> This cast is as needed as adding parentheses around the parameters
>> inside the definition of some function-like macro. Most of the time, you
>> can perfectly live without them but in some specific usages unexpected
>> patterns of behavior occur then you struggle debugging to fix the issue:
>>
>> #define MULT(a, b)	(a * b) /* instead of ((a) * (b)) */
>>
>> int a;
>>
>> a = MULT(2, 3); /* OK */
>> a = MULT(2 * 4, 8); /* OK */
>> a = MULT(2 + 4, 8); /* What result do you expect ? */
> 
> I think this is really completely irrelevant to my question. Besides,
> this is quite distracting and it took me quite a while to figure out
> what message you're trying to convey is.
>

I put I short answer first because I know you're busy and you don't like
to read long mail. Then I provided comparison with something I think
similar to illustrate my point. This was not supposed to offend you.
Indeed, you not the first one wondering about that, for instance
I've already talked about this issue with Ludovic here:
https://patchwork.ozlabs.org/patch/750084/

So this was not a personal attack against you. Sorry if you felt like it
was. There are other people on the mailing list, so I thought some of
them might have been interested by the explanation.


>> So it's always a good habit to cast into some unsigned integer type when
>> working with bitmasks/bitfields as it's always a good habit to add
>> parentheses around macro parameters: it's safer and it avoids falling
>> into some nasty traps!
>>
>> Please have a look at the definitions of GENMASK() and BIT() macros in
>> include/linux/bitops.h: they are defined using unsigned integers and
>> there are technical reasons behind that.
> 
> Yes, I had a look. They use the UL suffix for constants , which means
> the result is unsigned long, which according to C99 spec is at least
> 32bit datatype.
> 
> In your case, you use datatype which is explicitly 32bit only, so there
> is difference.
> 
> If you're adamant about the casts, unsigned long is probably what you
> want here .
>

'unsigned long' instead of 'u32': is this what you suggest?

Can you please explain what you have in mind because I don't understand
your point. What issue do you think it would fix?

The bit positions used in this patch are all below 32 so if the enum is
truncated to its 32 LSb, no information will be lost.


>> # Technical explanation:
>>
>> First thing to care about: the enum
>>
>> An enum is guaranteed to be represented by an integer, but the actual
>> type (and its signedness) is implementation-dependent.
> 
> So the conclusion about enum is ... ?
> 
>> Second thing to know: the >> operator
>>
>> The >> operator is perfectly defined when applied on an unsigned integer
>> whereas its output depends on the implementation with a signed integer
>> operand.
>> In practice, in most cases, the >> on signed integer performs an
>> arithmetic right shift and not a logical right shift as most people expect.
>>
>> signed int v1 = 0x80000000;
> 
> Isn't such overflow undefined anyway ?
>

overflow? why? v1 is the negative number with the highest absolute value
which can be encoded with 32 bits.

Or maybe I misunderstood your question?


>> (v1 >>  1) == 0xC0000000
>> (v1 >> 31) == 0xFFFFFFFF
>>
>>
>> unsigned int v2 = 0x80000000U;
>>
>> (v2 >>  1) == 0x40000000U
>> (v2 >> 31) == 0x00000001U
>>
>>
>> Then, if you define some bitmask/bitfield including the 31st bit:
>>
>> #define FIELD_SHIFT	30
>> #define FIELD_MASK	GENMASK(31, 30)
>> #define FIELD_VAL0	(0x0U << FIELD_SHIFT)
>> #define FIELD_VAL1	(0x1U << FIELD_SHIFT)
>> #define FIELD_VAL2	(0x2U << FIELD_SHIFT)
>> #define FIELD_VAL3	(0x3U << FIELD_SHIFT)
>>
>>
>> enum spi_nor_protocol {
>> 	PROTO0 = [...],
>>
>> 	PROTO42 = FIELD_VAL2 | [...],
>> };
>>
>> enum spi_nor_protocol proto = PROTO42
>> u8 val;
>>
>> val = (proto & FIELD_MASK) >> FIELD_SHIFT;
>> if (val == 0x2U) {
>> 	/*
>> 	 * We may not reach this point as expected because val
>> 	 * may be equal to 0xFEU depending on the implementation.
>> 	 */
>> }
> 
> And if you first shift and then mask ? Then you have no problem , yes?
> 

Yes you can but in this later case you will have to use a 2nd mask

#define FIELD_MASK2	GENMASK(1, 0)

(proto >> FIELD_SHIFT) & FIELDS_MASK2

However some people could then wonder when FIELD_MASK2 should be used
instead of FIELD_MASK. They are likely to wonder why they can't use
FIELD_MASK in all cases? I think this would lead to even more confusion.

The solution base on an explicit cast to a unsigned integer is discreet
and harmless, no performance penalty. Don't you agree?

If you choose FIELD_MASK2, you still need FIELD_MASK for tests like this
one:

u32 reg;

if ((reg & FIELD_MASK) == FIELD_VAL2)
	[...]

Of course you could do it with FIELD_MASK2 too:

if (((reg >> FIELD_SHIFT) & FIELD_MASK2) == (FIELD_VAL2 >> FIELD_SHIFT))
	 [...]

We can always find different ways to do things but is it worth spending
so much time discussing on this? ;)

You asked me a question then I answer you with some explanations because
I think this is more polite, respectful and useful for everybody than
one word answer (yes / no).
This was not intended to offend you :)


So to conclude, do you see any bug, regression or blocking point that
would justify not to include those patches in the PR?

In a previous mail, you've have written "Looks good otherwise."
Can we close that chapter then?

Best regards,

Cyrille

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