Fix SD info64 : info.usedBytes calculation giving weird result

[luc-github]

No description provided.

[earlephilhower]

When I manually factor things I think this change is mathematically the same as the existing code.

info.usedBytes = info.totalBytes - (_fs.vol()->freeClusterCount() * _fs.vol()->bytesPerCluster());
info.totalBytes =_fs.vol()->clusterCount() * info.blockSize;
info.blockSize = _fs.vol()->bytesPerCluster();

So,

info.usedBytes = (_fs.vol()->clusterCount() * _fs.vol()->bytesPerCluster()) - (_fs.vol()->freeClusterCount() * _fs.vol()->bytesPerCluster());

Factor out the bpc and you get

info.usedBytes = (_fs.vol()->clusterCount() - _fs.vol()->freeClusterCount() ) * _fs.vol()->bytesPerCluster();

which is the patch's rewrite, no?

[luc-github]

I do agree about calculation , but the fact is the current does not give the correct result.
Seems overflow happen.

Could be a type issue when doing calculation ?

[luc-github]

with figures it is may be more clear:

#include <SPI.h>
#include <SD.h>
#include <SDFS.h>
void setup(){
    Serial.begin(115200);
    if(!SD.begin(5)){
        Serial.println("Card Mount Failed");
        return;
    }
	FSInfo64 info;
	 if (!SDFS.info64(info)) {
		 Serial.println("Size unkown");
            return ;
        }
    Serial.printf("\nTotal space: %lluMB\n", info.totalBytes / (1024 * 1024));
    Serial.printf("Used space: %lluMB\n", info.usedBytes / (1024 * 1024));
}

void loop(){}

Without PR
Total space: 3760MB
Used space: 4929MB =>Wrong

With PR
Total space: 3760MB
Used space: 833MB =>Correct

[earlephilhower]

Thanks for giving the example. You're probably right, there is some type promotion issue and 32b overflow in the existing code.

[mcspr]

Maybe we would want explicit casts to u64? There are mixed i32, u16 and u32, while we expect the result to be u64-wide and not u32 cast into one in the end
Plus, freeClusterCount() may return -1