Re: [PATCH] Input: applespi - avoid wasting some memory

[Christophe JAILLET]

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@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

Got it. I'll keep away from it.

Thanks for the feed-back and explanation.

CJ