From: Willy Tarreau <w@1wt.eu>
To: Isaac Lleida <illeida@openaliasbox.org>
Cc: willy@meta-x.org, gregkh@linuxfoundation.org,
marius.gorski@gmail.com, armand.bastien@laposte.net,
domdevlin@free.fr, sudipm.mukherjee@gmail.com,
devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org,
kernel-janitors@vger.kernel.org
Subject: Re: [PATCH v3] staging: panel: change struct bits to a bit array
Date: Sun, 15 Mar 2015 08:57:50 +0100 [thread overview]
Message-ID: <20150315075750.GC9611@1wt.eu> (raw)
In-Reply-To: <1426328083-9183-1-git-send-email-illeida@openaliasbox.org>
Hello,
On Sat, Mar 14, 2015 at 11:14:43AM +0100, Isaac Lleida wrote:
> This path implements a bit array representing the LCD signal states instead of the old "struct bits", which used char to represent a single bit. This will reduce the memory usage.
>
> Signed-off-by: Isaac Lleida <illeida@openaliasbox.org>
> ---
> v3: some more stupid errors I introduced in last patch fixed.
Let me guess, you have never tested this patch series, right ?
I have not checked the code with the patch applied, but I'm seeing
this :
> /*
> + * LCD signal states
> + */
> +#define LCD_BIT_E_MASK 0x1 /* E (data latch on falling edge) */
> +#define LCD_BIT_RS_MASK 0x2 /* RS (0 = cmd, 1 = data) */
> +#define LCD_BIT_RW_MASK 0x4 /* R/W (0 = W, 1 = R) */
> +#define LCD_BIT_BL_MASK 0x8 /* backlight (0 = off, 1 = on) */
> +#define LCD_BIT_CL_MASK 0x10 /* clock (latch on rising edge) */
> +#define LCD_BIT_DA_MASK 0x20 /* data */
> +
> +/*
> + * Bit array operations
> + */
> +#define BIT_ON(b, m) (b |= m)
> +#define BIT_OFF(b, m) (b &= ~m)
> +#define BIT_CHK(b, m) (b & m)
Then this :
> - val |= lcd_bits[LCD_PORT_D][LCD_BIT_E][bits.e]
> - | lcd_bits[LCD_PORT_D][LCD_BIT_RS][bits.rs]
> - | lcd_bits[LCD_PORT_D][LCD_BIT_RW][bits.rw]
> - | lcd_bits[LCD_PORT_D][LCD_BIT_BL][bits.bl]
> - | lcd_bits[LCD_PORT_D][LCD_BIT_CL][bits.cl]
> - | lcd_bits[LCD_PORT_D][LCD_BIT_DA][bits.da];
> + val |= lcd_bits[LCD_PORT_D][LCD_BIT_E][BIT_CHK(bits, LCD_BIT_E_MASK)]
> + | lcd_bits[LCD_PORT_D][LCD_BIT_RS][BIT_CHK(bits, LCD_BIT_RS_MASK)]
> + | lcd_bits[LCD_PORT_D][LCD_BIT_RW][BIT_CHK(bits, LCD_BIT_RW_MASK)]
> + | lcd_bits[LCD_PORT_D][LCD_BIT_BL][BIT_CHK(bits, LCD_BIT_BL_MASK)]
> + | lcd_bits[LCD_PORT_D][LCD_BIT_CL][BIT_CHK(bits, LCD_BIT_CL_MASK)]
> + | lcd_bits[LCD_PORT_D][LCD_BIT_DA][BIT_CHK(bits, LCD_BIT_DA_MASK)];
So as you can see, previously lcd_bits[x][y][z] was used
with values 0 or 1 for bits, and now it's being used with
values of z matching the bit position in your array. So
it seems to me that it's a significant overflow. Thus I
think you should modify your macro this way :
#define BIT_CHK(b, m) (!!(b & m))
That said, given the amount of sensitive changes, and the
risk of breakage (as was apparently done here), you *must*
absolutely test you changes on real hardware before risking
to break the driver for every user. Also, the fact that you
needed 3 iterations of this patch after discovering breakage
is clearly a sign of the fact that you need to test it.
Thanks,
Willy
prev parent reply other threads:[~2015-03-15 7:58 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-14 10:14 [PATCH v3] staging: panel: change struct bits to a bit array Isaac Lleida
2015-03-15 7:57 ` Willy Tarreau [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20150315075750.GC9611@1wt.eu \
--to=w@1wt.eu \
--cc=armand.bastien@laposte.net \
--cc=devel@driverdev.osuosl.org \
--cc=domdevlin@free.fr \
--cc=gregkh@linuxfoundation.org \
--cc=illeida@openaliasbox.org \
--cc=kernel-janitors@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=marius.gorski@gmail.com \
--cc=sudipm.mukherjee@gmail.com \
--cc=willy@meta-x.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).