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@xxxxxxxxxx>
> ---
> 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
