[PATCH] Input: applespi - avoid wasting some memory

[PATCH] Input: applespi - avoid wasting some memory

[Christophe JAILLET]

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;
@@ -1659,21 +1659,6 @@ static int applespi_probe(struct spi_device *spi)
 	/* store the driver data */
 	spi_set_drvdata(spi, applespi);
 
-	/* create our buffers */
-	applespi->tx_buffer = devm_kmalloc(&spi->dev, APPLESPI_PACKET_SIZE,
-					   GFP_KERNEL);
-	applespi->tx_status = devm_kmalloc(&spi->dev, APPLESPI_STATUS_SIZE,
-					   GFP_KERNEL);
-	applespi->rx_buffer = devm_kmalloc(&spi->dev, APPLESPI_PACKET_SIZE,
-					   GFP_KERNEL);
-	applespi->msg_buf = devm_kmalloc_array(&spi->dev, MAX_PKTS_PER_MSG,
-					       APPLESPI_PACKET_SIZE,
-					       GFP_KERNEL);
-
-	if (!applespi->tx_buffer || !applespi->tx_status ||
-	    !applespi->rx_buffer || !applespi->msg_buf)
-		return -ENOMEM;
-
 	/* set up our spi messages */
 	applespi_setup_read_txfrs(applespi);
 	applespi_setup_write_txfrs(applespi);
-- 
2.34.1

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

[Johan Hovold]

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

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@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