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Date:   Mon, 19 Sep 2022 09:02:46 +0200
From:   Johan Hovold <johan@kernel.org>
To:     Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc:     Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org,
        linux-input@vger.kernel.org
Subject: Re: [PATCH] Input: applespi - avoid wasting some memory
Message-ID: <YygUFr5cSpZhYKOA@hovoldconsulting.com>
References: <0db94f84920663f3bd45a73e2ae73950627a377f.1663506472.git.christophe.jaillet@wanadoo.fr>
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On Sun, Sep 18, 2022 at 03:08:17PM +0200, Christophe JAILLET wrote:
> When the 'struct applespi_data' structure is allocated at the beginning of
> applespi_probe(), 2504 bytes are allocated.
> 
> Because of the way memory is allocated, it ends to a 4096 bytes allocation.
> So, about 1500 bytes are wasted.
> 
> Later in this function, when 'tx_buffer', 'tx_status', 'rx_buffer' and
> 'msg_buf' are allocated, 256, 4, 256 and 512 bytes are requested (~1 ko).
> A devm_ memory allocation has a small overhead of 40 bytes. So, for the
> same reason as above, it ends to allocate 512, 64, 512 and 1024 (~2 ko).
> 
> All that said, defining these 4 arrays as part of 'struct applespi_data'
> saves 2 ko of runtime memory.
> 
> 3504 bytes are now requested, and 4096 really allocated. All these 4
> arrays fit in the 'wasted' memory of the first allocation.
> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
> Compile tested only.
> ---
>  drivers/input/keyboard/applespi.c | 23 ++++-------------------
>  1 file changed, 4 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/input/keyboard/applespi.c b/drivers/input/keyboard/applespi.c
> index fab5473ae5da..bee4ccfa2b05 100644
> --- a/drivers/input/keyboard/applespi.c
> +++ b/drivers/input/keyboard/applespi.c
> @@ -373,11 +373,11 @@ struct applespi_data {
>  	struct input_dev		*keyboard_input_dev;
>  	struct input_dev		*touchpad_input_dev;
>  
> -	u8				*tx_buffer;
> -	u8				*tx_status;
> -	u8				*rx_buffer;
> +	u8				tx_buffer[APPLESPI_PACKET_SIZE];
> +	u8				tx_status[APPLESPI_STATUS_SIZE];
> +	u8				rx_buffer[APPLESPI_PACKET_SIZE];
>  
> -	u8				*msg_buf;
> +	u8				msg_buf[MAX_PKTS_PER_MSG * APPLESPI_PACKET_SIZE];
>  	unsigned int			saved_msg_len;
>  
>  	struct applespi_tp_info		tp_info;

This kind of change is generally broken in case DMA can be involved.

Allocating the transfer buffers separately makes sure that alignment
requirements are met and avoids hard-to-debug memory corruption issues.

Johan