linux-kernel - Re: [PATCH v1 10/15] auxdisplay: linedisp: Provide a small buffer in the struct linedisp

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Date: Mon, 12 Feb 2024 09:25:00 +0100
From: Robin van der Gracht <robin@...tonic.nl>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, Miguel Ojeda
 <ojeda@...nel.org>, Rob Herring <robh@...nel.org>, Krzysztof Kozlowski
 <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>,
 Paul Burton <paulburton@...nel.org>, Geert Uytterhoeven
 <geert+renesas@...der.be>
Subject: Re: [PATCH v1 10/15] auxdisplay: linedisp: Provide a small buffer
 in the struct linedisp

Hi Andy,

Thank you for your patches.

See inline comment below.

On Thu,  8 Feb 2024 20:48:08 +0200
Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:

> There is a driver that uses small buffer for the string, when we
> add a new one, we may avoid duplication and use one provided by
> the line display library. Allow user to skip buffer pointer when
> registering a device.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>  drivers/auxdisplay/line-display.c | 4 ++--
>  drivers/auxdisplay/line-display.h | 4 ++++
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/auxdisplay/line-display.c b/drivers/auxdisplay/line-display.c
> index da47fc59f6cb..a3c706e1739d 100644
> --- a/drivers/auxdisplay/line-display.c
> +++ b/drivers/auxdisplay/line-display.c
> @@ -330,8 +330,8 @@ int linedisp_register(struct linedisp *linedisp, struct device *parent,
>  	linedisp->dev.parent = parent;
>  	linedisp->dev.type = &linedisp_type;
>  	linedisp->ops = ops;
> -	linedisp->buf = buf;
> -	linedisp->num_chars = num_chars;
> +	linedisp->buf = buf ? buf : linedisp->curr;
> +	linedisp->num_chars = buf ? num_chars : min(num_chars, LINEDISP_DEFAULT_BUF_SZ);

It's not a big buffer, but now it's always there even if it's not used.
And even if it's used, it might be only partially used.
Why not used a malloc instead?

>  	linedisp->scroll_rate = DEFAULT_SCROLL_RATE;
>  
>  	err = ida_alloc(&linedisp_id, GFP_KERNEL);
> diff --git a/drivers/auxdisplay/line-display.h b/drivers/auxdisplay/line-display.h
> index 65d782111f53..4c354b8f376e 100644
> --- a/drivers/auxdisplay/line-display.h
> +++ b/drivers/auxdisplay/line-display.h
> @@ -54,12 +54,15 @@ struct linedisp_ops {
>  	void (*update)(struct linedisp *linedisp);
>  };
>  
> +#define LINEDISP_DEFAULT_BUF_SZ		8u
> +
>  /**
>   * struct linedisp - character line display private data structure
>   * @dev: the line display device
>   * @id: instance id of this display
>   * @timer: timer used to implement scrolling
>   * @ops: character line display operations
> + * @curr: fallback buffer for the string
>   * @buf: pointer to the buffer for the string currently displayed
>   * @message: the full message to display or scroll on the display
>   * @num_chars: the number of characters that can be displayed
> @@ -73,6 +76,7 @@ struct linedisp {
>  	struct timer_list timer;
>  	const struct linedisp_ops *ops;
>  	struct linedisp_map *map;
> +	char curr[LINEDISP_DEFAULT_BUF_SZ];
>  	char *buf;
>  	char *message;
>  	unsigned int num_chars;