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From: Trent Piepho <tpiepho@impinj.com>
To: "gustavo@embeddedor.com" <gustavo@embeddedor.com>,
	"bcm-kernel-feedback-list@broadcom.com"
	<bcm-kernel-feedback-list@broadcom.com>,
	"stefan.wahren@i2se.com" <stefan.wahren@i2se.com>,
	"f.fainelli@gmail.com" <f.fainelli@gmail.com>,
	"broonie@kernel.org" <broonie@kernel.org>,
	"eric@anholt.net" <eric@anholt.net>,
	"rjui@broadcom.com" <rjui@broadcom.com>,
	"sbranden@broadcom.com" <sbranden@broadcom.com>
Cc: "linux-spi@vger.kernel.org" <linux-spi@vger.kernel.org>,
	"linux-rpi-kernel@lists.infradead.org"
	<linux-rpi-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"garsilva@embeddedor.com" <garsilva@embeddedor.com>
Subject: Re: [PATCH] spi: bcm2835aux: use 64-bit arithmetic instead of 32-bit
Date: Mon, 12 Feb 2018 19:31:46 +0000	[thread overview]
Message-ID: <1518463905.25398.281.camel@impinj.com> (raw)
In-Reply-To: <20180207160002.GA9292@embeddedgus>

On Wed, 2018-02-07 at 10:00 -0600, Gustavo A. R. Silva wrote:
> Add suffix ULL to constant 9 in order to give the compiler complete
> information about the proper arithmetic to use. Notice that this
> constant is used in a context that expects an expression of type
> unsigned long long (64 bits, unsigned).
> 
> The expression tfr->len * 9 * 1000000 is currently being evaluated
> using 32-bit arithmetic.
> 
> Addresses-Coverity-ID: 1339619
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
>  drivers/spi/spi-bcm2835aux.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi-bcm2835aux.c b/drivers/spi/spi-bcm2835aux.c
> index 7428091..a768c23 100644
> --- a/drivers/spi/spi-bcm2835aux.c
> +++ b/drivers/spi/spi-bcm2835aux.c
> @@ -363,7 +363,7 @@ static int bcm2835aux_spi_transfer_one(struct spi_master *master,
>  	 * chunk getting transferred - in our case the chunk size
>  	 * is 3 bytes, so we approximate this by 9 bits/byte
>  	 */
> -	xfer_time_us = tfr->len * 9 * 1000000;
> +	xfer_time_us = tfr->len * 9ULL * 1000000;
>  	do_div(xfer_time_us, spi_used_hz);
>  
>  	/* run in polling mode for short transfers */
	if (xfer_time_us < BCM2835_AUX_SPI_POLLING_LIMIT_US)

If one looks at the logic as a whole...

tfr->len * 9ULL * 100000 / spi_used_hz < 30

A little algebra can change that into:

tfr->len < spi_used_hz / 300000

So rather than add more 64-bit math, it can be removed entirely.  Both
the length and speed are 32 bits and there's no need to multiply them
into something larger than might need 64 bits.

WARNING: multiple messages have this Message-ID (diff)
From: Trent Piepho <tpiepho-cgc2CodaaHDQT0dZR+AlfA@public.gmane.org>
To: "gustavo-L1vi/lXTdts+Va1GwOuvDg@public.gmane.org"
	<gustavo-L1vi/lXTdts+Va1GwOuvDg@public.gmane.org>,
	"bcm-kernel-feedback-list-dY08KVG/lbpWk0Htik3J/w@public.gmane.org"
	<bcm-kernel-feedback-list-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>,
	"stefan.wahren-eS4NqCHxEME@public.gmane.org"
	<stefan.wahren-eS4NqCHxEME@public.gmane.org>,
	"f.fainelli-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org"
	<f.fainelli-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	"broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org"
	<broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	"eric-WhKQ6XTQaPysTnJN9+BGXg@public.gmane.org"
	<eric-WhKQ6XTQaPysTnJN9+BGXg@public.gmane.org>,
	"rjui-dY08KVG/lbpWk0Htik3J/w@public.gmane.org"
	<rjui-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>,
	"sbranden-dY08KVG/lbpWk0Htik3J/w@public.gmane.org"
	<sbranden-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>
Cc: "linux-spi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-spi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"linux-rpi-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org"
	<linux-rpi-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org>,
	"linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org"
	<linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org>,
	"garsilva-L1vi/lXTdts+Va1GwOuvDg@public.gmane.org"
	<garsilva-L1vi/lXTdts+Va1GwOuvDg@public.gmane.org>
Subject: Re: [PATCH] spi: bcm2835aux: use 64-bit arithmetic instead of 32-bit
Date: Mon, 12 Feb 2018 19:31:46 +0000	[thread overview]
Message-ID: <1518463905.25398.281.camel@impinj.com> (raw)
In-Reply-To: <20180207160002.GA9292@embeddedgus>

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset="utf-8", Size: 1755 bytes --]

On Wed, 2018-02-07 at 10:00 -0600, Gustavo A. R. Silva wrote:
> Add suffix ULL to constant 9 in order to give the compiler complete
> information about the proper arithmetic to use. Notice that this
> constant is used in a context that expects an expression of type
> unsigned long long (64 bits, unsigned).
> 
> The expression tfr->len * 9 * 1000000 is currently being evaluated
> using 32-bit arithmetic.
> 
> Addresses-Coverity-ID: 1339619
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
>  drivers/spi/spi-bcm2835aux.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi-bcm2835aux.c b/drivers/spi/spi-bcm2835aux.c
> index 7428091..a768c23 100644
> --- a/drivers/spi/spi-bcm2835aux.c
> +++ b/drivers/spi/spi-bcm2835aux.c
> @@ -363,7 +363,7 @@ static int bcm2835aux_spi_transfer_one(struct spi_master *master,
>  	 * chunk getting transferred - in our case the chunk size
>  	 * is 3 bytes, so we approximate this by 9 bits/byte
>  	 */
> -	xfer_time_us = tfr->len * 9 * 1000000;
> +	xfer_time_us = tfr->len * 9ULL * 1000000;
>  	do_div(xfer_time_us, spi_used_hz);
>  
>  	/* run in polling mode for short transfers */
	if (xfer_time_us < BCM2835_AUX_SPI_POLLING_LIMIT_US)

If one looks at the logic as a whole...

tfr->len * 9ULL * 100000 / spi_used_hz < 30

A little algebra can change that into:

tfr->len < spi_used_hz / 300000

So rather than add more 64-bit math, it can be removed entirely.  Both
the length and speed are 32 bits and there's no need to multiply them
into something larger than might need 64 bits.
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WARNING: multiple messages have this Message-ID (diff)
From: tpiepho@impinj.com (Trent Piepho)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] spi: bcm2835aux: use 64-bit arithmetic instead of 32-bit
Date: Mon, 12 Feb 2018 19:31:46 +0000	[thread overview]
Message-ID: <1518463905.25398.281.camel@impinj.com> (raw)
In-Reply-To: <20180207160002.GA9292@embeddedgus>

On Wed, 2018-02-07 at 10:00 -0600, Gustavo A. R. Silva wrote:
> Add suffix ULL to constant 9 in order to give the compiler complete
> information about the proper arithmetic to use. Notice that this
> constant is used in a context that expects an expression of type
> unsigned long long (64 bits, unsigned).
> 
> The expression tfr->len * 9 * 1000000 is currently being evaluated
> using 32-bit arithmetic.
> 
> Addresses-Coverity-ID: 1339619
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
>  drivers/spi/spi-bcm2835aux.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi-bcm2835aux.c b/drivers/spi/spi-bcm2835aux.c
> index 7428091..a768c23 100644
> --- a/drivers/spi/spi-bcm2835aux.c
> +++ b/drivers/spi/spi-bcm2835aux.c
> @@ -363,7 +363,7 @@ static int bcm2835aux_spi_transfer_one(struct spi_master *master,
>  	 * chunk getting transferred - in our case the chunk size
>  	 * is 3 bytes, so we approximate this by 9 bits/byte
>  	 */
> -	xfer_time_us = tfr->len * 9 * 1000000;
> +	xfer_time_us = tfr->len * 9ULL * 1000000;
>  	do_div(xfer_time_us, spi_used_hz);
>  
>  	/* run in polling mode for short transfers */
	if (xfer_time_us < BCM2835_AUX_SPI_POLLING_LIMIT_US)

If one looks@the logic as a whole...

tfr->len * 9ULL * 100000 / spi_used_hz < 30

A little algebra can change that into:

tfr->len < spi_used_hz / 300000

So rather than add more 64-bit math, it can be removed entirely.  Both
the length and speed are 32 bits and there's no need to multiply them
into something larger than might need 64 bits.

  parent reply	other threads:[~2018-02-12 19:31 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-07 16:00 [PATCH] spi: bcm2835aux: use 64-bit arithmetic instead of 32-bit Gustavo A. R. Silva
2018-02-07 16:00 ` Gustavo A. R. Silva
2018-02-08  8:22 ` Eric Anholt
2018-02-08  8:22   ` Eric Anholt
2018-02-12 17:57   ` Gustavo A. R. Silva
2018-02-12 17:57     ` Gustavo A. R. Silva
2018-02-08  9:54 ` Ard Biesheuvel
2018-02-08  9:54   ` Ard Biesheuvel
2018-02-08  9:54   ` Ard Biesheuvel
2018-02-12 18:04   ` Gustavo A. R. Silva
2018-02-12 18:04     ` Gustavo A. R. Silva
2018-02-12 18:45     ` Ard Biesheuvel
2018-02-12 18:45       ` Ard Biesheuvel
2018-02-12 19:10       ` Florian Fainelli
2018-02-12 19:10         ` Florian Fainelli
2018-02-12 19:10         ` Florian Fainelli
2018-02-12 19:11         ` Ard Biesheuvel
2018-02-12 19:11           ` Ard Biesheuvel
2018-02-12 19:11           ` Ard Biesheuvel
2018-02-12 19:14           ` Florian Fainelli
2018-02-12 19:14             ` Florian Fainelli
2018-02-12 19:14             ` Florian Fainelli
2018-02-12 19:31 ` Trent Piepho [this message]
2018-02-12 19:31   ` Trent Piepho
2018-02-12 19:31   ` Trent Piepho
2018-02-12 19:38   ` [PATCH] spi: bcm2835aux: Avoid 64-bit arithmetic in xfer len calc Trent Piepho
2018-02-12 19:38     ` Trent Piepho
2018-02-14 16:04     ` Mark Brown
2018-02-14 16:04       ` Mark Brown
2018-02-14 16:04       ` Mark Brown
2018-02-14 16:28     ` Applied "spi: bcm2835aux: Avoid 64-bit arithmetic in xfer len calc" to the spi tree Mark Brown
2018-02-14 16:28       ` Mark Brown

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