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From: "Andreas Färber" <afaerber@suse.de>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-realtek-soc@lists.infradead.org,
	linux-leds@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-spi <linux-spi@vger.kernel.org>,
	Mark Brown <broonie@kernel.org>,
	Jacek Anaszewski <jacek.anaszewski@gmail.com>,
	Pavel Machek <pavel@ucw.cz>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Dan Murphy <dmurphy@ti.com>
Subject: Re: [RFC 04/25] spi: gpio: Implement LSB First bitbang support
Date: Thu, 12 Dec 2019 16:14:59 +0100
Message-ID: <9b4b6287-c1d9-1b41-88a8-7ac9fe222642@suse.de> (raw)
In-Reply-To: <CAMuHMdWdxJ9AaWhyCW-u8fCpXSDCPd-D6Dx129SF5nRssZsK=g@mail.gmail.com>

Hi Geert,

Am 12.12.19 um 09:40 schrieb Geert Uytterhoeven:
> On Thu, Dec 12, 2019 at 4:41 AM Andreas Färber <afaerber@suse.de> wrote:
>> Add support for slave DT property spi-lsb-first, i.e., SPI_LSB_FIRST mode.
>>
>> Duplicate the inline helpers bitbang_txrx_be_cpha{0,1} as LE versions.
>> Make checkpatch.pl happy by changing "unsigned" to "unsigned int".
>>
>> Conditionally call them from all the spi-gpio txrx_word callbacks.
>>
>> Signed-off-by: Andreas Färber <afaerber@suse.de>
> 
> Thanks for your patch!

NP. I prefer fixing issues at the source over awkward workarounds. :)

>> --- a/drivers/spi/spi-gpio.c
>> +++ b/drivers/spi/spi-gpio.c
>> @@ -135,25 +135,37 @@ static inline int getmiso(const struct spi_device *spi)
>>  static u32 spi_gpio_txrx_word_mode0(struct spi_device *spi,
>>                 unsigned nsecs, u32 word, u8 bits, unsigned flags)
>>  {
>> -       return bitbang_txrx_be_cpha0(spi, nsecs, 0, flags, word, bits);
>> +       if (unlikely(spi->mode & SPI_LSB_FIRST))
>> +               return bitbang_txrx_le_cpha0(spi, nsecs, 0, flags, word, bits);
>> +       else
>> +               return bitbang_txrx_be_cpha0(spi, nsecs, 0, flags, word, bits);
>>  }
> 
> Duplicating all functions sounds a bit wasteful to me.

Two functions duplicated, eight function calls duplicated.

> What about reverting the word first, and calling the normal functions?
> 
>     if (unlikely(spi->mode & SPI_LSB_FIRST)) {
>             if (bits <= 8)
>                     word = bitrev8(word) >> (bits - 8);
>             else if (bits <= 16)
>                     word = bitrev16(word) >> (bits - 16);
>             else
>                     word = bitrev32(word) >> (bits - 32);
>     }
>     return bitbang_txrx_be_cpha0(spi, nsecs, 0, flags, word, bits);

Hm, wasn't aware of those helpers, so I opted not to loop over the bits
for reversing myself, as Thomas Gleixner disliked bit loops in irqchip.
Performance appeared to be a concern here, too.

Eight functions duplicated then. I don't like that - at least we should
pack that into an inline helper or macro, to not copy&paste even more
lines around. Who knows, maybe we'll get 64-bit support at one point?

Do you think it would be acceptable to instead encapsulate this inside
the _be inline helpers? That would lead the name ad absurdum, of course,
but we would then need to do it only twice, not eight times.

However, either way would seem to make the LSB code path slower than MSB
due to the prepended reversal.

Delays are already stubbed out, with open TODOs for further inlining
functions that are being dispatched today.

So from that angle I don't see a better way than either duplicating the
functions or using some macro magic to #include the header twice. If we
wanted to go down that path, we could probably de-duplicate the existing
two functions, too, but I was trying to err on the cautious side, since
I don't have setups to test all four code paths myself (and a ton of
more relevant but less fun patches to flush out ;)).

Regards,
Andreas

-- 
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
GF: Felix Imendörffer
HRB 36809 (AG Nürnberg)

  reply index

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-12  3:39 [RFC 00/25] arm64: realtek: Add Xnano X5 and implement TM1628/FD628/AiP1618 LED controllers Andreas Färber
2019-12-12  3:39 ` [RFC 01/25] dt-bindings: vendor-prefixes: Add Xnano Andreas Färber
2019-12-19 22:26   ` Rob Herring
2019-12-12  3:39 ` [RFC 02/25] dt-bindings: arm: realtek: Add Xnano X5 Andreas Färber
2019-12-19 22:27   ` Rob Herring
2019-12-12  3:39 ` [RFC 03/25] arm64: dts: realtek: rtd1295: " Andreas Färber
2019-12-12  3:39 ` [RFC 04/25] spi: gpio: Implement LSB First bitbang support Andreas Färber
2019-12-12  8:40   ` Geert Uytterhoeven
2019-12-12 15:14     ` Andreas Färber [this message]
2019-12-12 17:19       ` Mark Brown
2019-12-12 21:08         ` Andreas Färber
2019-12-13 11:42           ` Mark Brown
2019-12-12  3:39 ` [RFC 05/25] dt-bindings: vendor-prefixes: Add Titan Micro Electronics Andreas Färber
2019-12-19 22:31   ` Rob Herring
2019-12-12  3:39 ` [RFC 06/25] dt-bindings: leds: Add Titan Micro Electronics TM1628 Andreas Färber
2019-12-19 23:04   ` Rob Herring
2019-12-12  3:39 ` [RFC 07/25] " Andreas Färber
2019-12-14  9:48   ` Andreas Färber
2019-12-12  3:39 ` [RFC 08/25] arm64: dts: realtek: rtd129x-zidoo-x9s: Add TM1628 LED controller Andreas Färber
2019-12-12  3:39 ` [RFC 09/25] arm64: dts: realtek: rtd1295-zidoo-x9s: Add regular LEDs to TM1628 Andreas Färber
2019-12-12  3:39 ` [RFC 10/25] dt-bindings: vendor-prefixes: Add Fuda Hisi Microelectronics Andreas Färber
2019-12-19 23:04   ` Rob Herring
2019-12-12  3:39 ` [RFC 11/25] dt-bindings: leds: tm1628: Add Fuda Hisi Microelectronics FD628 Andreas Färber
2019-12-19 23:05   ` Rob Herring
2019-12-12  3:39 ` [RFC 12/25] " Andreas Färber
2019-12-12  3:39 ` [RFC 13/25] arm64: dts: realtek: rtd1295-xnano-x5: Add FD628 LED controller Andreas Färber
2019-12-12  3:39 ` [RFC 14/25] arm64: dts: realtek: rtd1295-xnano-x5: Add regular LEDs to FD628 Andreas Färber
2019-12-21 20:21   ` Pavel Machek
2019-12-12  3:39 ` [RFC 15/25] dt-bindings: vendor-prefixes: Add Fude Microelectronics Andreas Färber
2019-12-19 23:05   ` Rob Herring
2019-12-12  3:39 ` [RFC 16/25] dt-bindings: leds: tm1628: Add Fude Microelectronics AiP1618 Andreas Färber
2019-12-19 23:06   ` Rob Herring
2019-12-12  3:39 ` [RFC 17/25] leds: tm1628: Prepare " Andreas Färber
2019-12-21 19:55   ` Andreas Färber
2019-12-12  3:39 ` [RFC 18/25] dt-bindings: leds: tm1628: Define display child nodes Andreas Färber
2019-12-12  3:39 ` [RFC 19/25] leds: tm1628: Add 7-segment display support Andreas Färber
2019-12-12  8:33   ` Geert Uytterhoeven
2019-12-12 14:10     ` Andreas Färber
2019-12-21 20:23   ` Pavel Machek
2019-12-12  3:39 ` [RFC 20/25] arm64: dts: realtek: rtd1295-zidoo-x9s: Add display to TM1628 Andreas Färber
2019-12-12  3:39 ` [RFC 21/25] arm64: dts: realtek: rtd1295-xnano-x5: Add display to FD628 Andreas Färber
2019-12-12  3:39 ` [RFC 22/25] leds: tm1826: Add combined glyph support Andreas Färber
2019-12-21 20:27   ` Pavel Machek
2019-12-21 20:41     ` Andreas Färber
2019-12-21 21:04       ` Pavel Machek
2019-12-21 21:49         ` Andreas Färber
     [not found]           ` <CANiq72nA9OLa0SjY8W055J_2A32tcp7S98SruKSdWH2dm25VKw@mail.gmail.com>
2019-12-22  3:14             ` Andreas Färber
2020-01-15 14:31               ` Miguel Ojeda
2019-12-12  3:39 ` [RFC 23/25] WIP: leds: tm1628: Prepare TM1628 keys Andreas Färber
2019-12-12  3:39 ` [RFC 24/25] WIP: leds: tm1628: Prepare FD628 keys Andreas Färber
2019-12-12  3:39 ` [RFC 25/25] WIP: leds: tm1628: Prepare AiP1618 keys Andreas Färber
2019-12-12 13:14 ` [RFC 00/25] arm64: realtek: Add Xnano X5 and implement TM1628/FD628/AiP1618 LED controllers Robin Murphy
2019-12-12 20:55   ` Andreas Färber
2019-12-13 14:07     ` Robin Murphy
2019-12-13 14:36       ` Geert Uytterhoeven
2019-12-21 18:20 ` Pavel Machek
2019-12-21 21:07   ` Andreas Färber
2020-01-15 13:34 ` Andreas Färber

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