linux-kernel - Re: [PATCH] Input: applespi

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Date:   Mon, 19 Sep 2022 19:33:30 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Johan Hovold <johan@...nel.org>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-input@...r.kernel.org
Subject: Re: [PATCH] Input: applespi - avoid wasting some memory

Le 19/09/2022 à 09:02, Johan Hovold a écrit :
> 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@...adoo.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
> 

Got it. I'll keep away from it.

Thanks for the feed-back and explanation.

CJ