From: Eddie James <eajames-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
To: Andy Shevchenko
<andy.shevchenko-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Eddie James <eajames-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>,
linux-spi <linux-spi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
Linux Kernel Mailing List
<linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
Joel Stanley <joel-U3u1mxZcP9KHXe+LvDLADg@public.gmane.org>,
Andrew Jeffery <andrew-zrmu5oMJ5Fs@public.gmane.org>
Subject: Re: [PATCH] spi: Add FSI-attached SPI controller driver
Date: Mon, 10 Feb 2020 14:05:17 -0600 [thread overview]
Message-ID: <3bbc98a7-838d-f57f-0478-a632ba68f500@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAHp75VduJn=kMuriGNAGk1ZrL4b2LpqEn5c8AC=PHFKrgoz=vg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
On 2/7/20 4:04 PM, Andy Shevchenko wrote:
> On Fri, Feb 7, 2020 at 11:04 PM Eddie James <eajames-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org> wrote:
>> On 2/7/20 2:34 PM, Andy Shevchenko wrote:
>>> On Fri, Feb 7, 2020 at 10:04 PM Eddie James <eajames-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org> wrote:
>>>> On 2/7/20 1:39 PM, Andy Shevchenko wrote:
>>>>> On Fri, Feb 7, 2020 at 9:28 PM Eddie James <eajames-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org> wrote:
>>>>>> On 2/5/20 9:51 AM, Andy Shevchenko wrote:
>>>>>>> On Tue, Feb 4, 2020 at 6:06 PM Eddie James <eajames-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org> wrote:
>>>>>>>> On 2/4/20 5:02 AM, Andy Shevchenko wrote:
>>>>>>>>> On Mon, Feb 3, 2020 at 10:33 PM Eddie James <eajames-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org> wrote:
>>>>>>>>>> On 1/30/20 10:37 AM, Andy Shevchenko wrote:
>>>>>>>>>>>> + for (i = 0; i < num_bytes; ++i)
>>>>>>>>>>>> + rx[i] = (u8)((in >> (8 * ((num_bytes - 1) - i))) & 0xffULL);
>>>>>>>>>>> Redundant & 0xffULL part.
>>>>>>> For me it looks like
>>>>>>>
>>>>>>> u8 tmp[8];
>>>>>>>
>>>>>>> put_unaligned_be64(in, tmp);
>>>>>>> memcpy(rx, tmp, num_bytes);
>>>>>>>
>>>>>>> put_unaligned*() is just a method to unroll the value to the u8 buffer.
>>>>>>> See, for example, linux/unaligned/be_byteshift.h implementation.
>>>>>> Unforunately it is not the same. put_unaligned_be64 will take the
>>>>>> highest 8 bits (0xff00000000000000) and move it into tmp[0]. Then
>>>>>> 0x00ff000000000000 into tmp[1], etc. This is only correct for this
>>>>>> driver IF my transfer is 8 bytes. If, for example, I transfer 5 bytes,
>>>>>> then I need 0x000000ff00000000 into tmp[0], 0x00000000ff000000 into
>>>>>> tmp[1], etc. So I think my current implementation is correct.
>>>>> Yes, I missed correction of the start address in memcpy(). Otherwise
>>>>> it's still the same what I was talking about.
>>>> I see now, yes, thanks.
>>>>
>>>> Do you think this is worth a v3? Perhaps put_unaligned is slightly more
>>>> optimized than the loop but there is more memory copy with that way too.
>>> I already forgot the entire context when this has been called. Can you
>>> summarize what the sequence(s) of num_bytes are expected usually.
>>>
>>> IIUC if packets small, less than 8 bytes, than num_bytes will be that value.
>>> Otherwise it will be something like 8 + 8 + 8 ... + tail. Is it
>>> correct assumption?
>>
>> Yes, it will typically be 8 + 8 +... remainder. Basically, on any RX,
>> the driver polls for the rx register full. Once full, it will read
>> however much data is left to be transferred. Since we use min(len, 8)
>> then we read 8 usually, until we get to the end.
> I asked that because we might have a better optimization, i.e, call
> directly put_unaligned_be64() when we know that length is 8 bytes. For
> the rest your approach might be simpler. Similar for the TX case.
I just tried to implement as you suggested but I realized something: The
value is already swapped from BE to CPU when the register is read in
fsi_spi_read_reg. It happens to work out correctly to use
put_unaligned_be64 on a LE CPU to flip the bytes here. But on a BE CPU,
this wouldn't be correct I think. Now I don't anticipate this driver
running on a BE CPU, but I think it is weird to flip it twice, and
better to do it manually here.
What do you think Andy?
Thanks,
Eddie
>
>>>>>>>>>>>> + return num_bytes;
>>>>>>>>>>>> +}
next prev parent reply other threads:[~2020-02-10 20:05 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-29 20:08 [PATCH] spi: Add FSI-attached SPI controller driver Eddie James
[not found] ` <1580328504-436-1-git-send-email-eajames-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>
2020-01-30 14:46 ` Mark Brown
2020-01-30 15:32 ` Eddie James
2020-01-30 16:37 ` Andy Shevchenko
2020-02-03 20:33 ` Eddie James
[not found] ` <29f6cc86-69ca-bc88-b6ae-2b1a24c0dae3-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
2020-02-04 11:02 ` Andy Shevchenko
[not found] ` <CAHp75Vf3NCkbw39E+d_nf+AyViG2o-u5HxrCjXXmbGk4LaFLog-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-02-04 16:06 ` Eddie James
[not found] ` <744f0019-8656-eec1-cb9a-7e70cd042587-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>
2020-02-05 15:51 ` Andy Shevchenko
[not found] ` <CAHp75VfOM5Rd3LRBtvyT96G=+J4KxTRoSVUcQTj+RxrGyZMMnQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-02-07 19:28 ` Eddie James
[not found] ` <90973143-bd0a-33cf-9eb8-a83be1a9b415-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
2020-02-07 19:39 ` Andy Shevchenko
[not found] ` <CAHp75Vf6HJw=SpK9_HUgcMaaabs5pZEybP4SS-gc1wz5GRcqeg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-02-07 20:04 ` Eddie James
[not found] ` <cd8a7279-beb2-39e6-58e6-6ea505665547-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
2020-02-07 20:34 ` Andy Shevchenko
[not found] ` <CAHp75VdO2+Kkgo1Nv3EJXTMqP6Y7XgLVXtRvURk6Zz0p=R9zNQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-02-07 20:59 ` Eddie James
[not found] ` <ca8a832e-ceeb-3ce1-9807-96ee41e0f166-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
2020-02-07 22:04 ` Andy Shevchenko
[not found] ` <CAHp75VduJn=kMuriGNAGk1ZrL4b2LpqEn5c8AC=PHFKrgoz=vg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-02-10 20:05 ` Eddie James [this message]
2020-02-10 20:33 ` Andy Shevchenko
[not found] ` <CAHp75VfpRV7UDMpPKo8Vu1PaOfLjUG24yUdkg8ip9=923cwarA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-02-10 20:50 ` Eddie James
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