From: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
To: Johan Hovold <johan@kernel.org>
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
Date: Mon, 19 Sep 2022 19:33:30 +0200 [thread overview]
Message-ID: <e4914aa5-3e42-235c-fd9e-65a907a92842@wanadoo.fr> (raw)
In-Reply-To: <YygUFr5cSpZhYKOA@hovoldconsulting.com>
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@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
>
Got it. I'll keep away from it.
Thanks for the feed-back and explanation.
CJ
prev parent reply other threads:[~2022-09-19 17:33 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-18 13:08 [PATCH] Input: applespi - avoid wasting some memory Christophe JAILLET
2022-09-19 7:02 ` Johan Hovold
2022-09-19 17:33 ` Christophe JAILLET [this message]
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